How to help my mom deal with estrangement from my sister but also set boundaries?

Guilt is the wrong G work. Try to switch it to "grief" and let the guilt belong to felons who do evil and deserve it.
You are grieving that you cannot do it all. You are exhausted from job, Mom and long drives.
Your Sister really doesn't figure in this to tell you the truth. She is out of the picture.
I don't think she is the reason for your Mom's becoming more dependent. Your Mom is quite simply getting more and more aged and dependent. It is a natural progression. And you may be enabling her neediness by acquiescing to her wishes. You have your own life. If your being 25 minutes away isn't good enough Mom should consider a move to LTC where she will be around others, and not so frightened to be alone. This will eventually come to us all.
You do not mention that Mom has any limitations physically or mentally at this time, so I am assuming, unless you inform us differently that she doesn't. Don't encourage or enable her dependence or her needing you before she has real needs.
I sure wish you the best. Let your Sis go on her merry way. We can't change others.

If she's in independent living she needs to live independently. How often do you visit her? If it's as much as once a day you need to cut back. You can't be her sole form of entertainment. She will only get more and more needy as time goes by, so set those boundaries now and stick with them.

You are being torn between the reasonable desire to shorten your commute and the unreasonable feeling to be everything to your mom. You can’t have both and something has to give. Your mom needs to accept that you are only 15 minutes further away. I don’t see why she is making such a big deal over that. Truly it makes no sense other than she wants to have control over what you do and fears she is losing that.
your bio states your mom lives in IL with some cognitive decline. This may be affecting her neediness. I would go ahead and move. Traffic commutes can be terribly wearing, use more fuel, more chance of an accident etc. since you work you obviously can’t see her that often. Set a schedule of visits if you must. Meet her for dinner at her IL once a week or take her out for a meal. But don’t work your entire existence around your mom's need to control. She will get over it and adapt. What other choice does she have?

2 issues are really at work here.

1 - Your work situation. You will need a retirement to meet your own needs when you are older. So, you need to what works to help you meet that goal. This appears to include a move closer to work. Do it.

2 - Your mom's needs. Yes, family squabbles do happen. People get their feelings hurt. Some people - most likely your sister - have mental health issues that go unresolved. Your mom seems to need you a lot emotionally. Are you with her every day, several times a day....? When you are with her, what do you do to help her: hygiene, toileting, assist with walking or moving...? If so, then it is probably time to get more people involved: family, friends, members of faith community, and/or paid help. You could always move your mother closer to you when you find an apartment.

3 - Your sister. The only way you can keep your sister from being "nasty" is to cut off communication. If you and your mom can handle her disagreeableness by staying objective, then you may be able to keep some lines of communication open. Don't expect your sister to change. I suggest reading any of the books about "boundaries" by Townsend and Cloud to help with devising your plan for dealing with your sister's behavior.

I think that you need to live where you are content. Help mom look into an independent or assisted living facility. Would she consider moving close to the apartment that you choose?

You are not responsible for your sister’s actions. I am so sorry that your mom is being blamed for your sister’s troubles.

Over time your mom will come to peace with this. Support her by listening and once you have fully listened, distract her and redirect toward something more positive.

I have a sister just like yours. She did not talk to my parents for about 4 years. Mom wept many times. Mom told everyone about this painful relationship. She was denied precious time with grandchildren. It was worse than a death because it was intentional. My sister wrote my parents unbelievably cruel and hateful letters during that time.

My mistake was trying to help them reconcile. (I was close to my sister - my mom used to warn me that being close to her would hurt me in the long run). There are places in the world where peace can just never be truly accomplished and it would have been better to accept that this was one of them.

My attempts at bringing them together just brought mom more distress and pain. It eventually brought me pain too, as mom correctly predicted.

Mom chose to write my sister out of her will. As Mom’s executor, I was vigorously sued. (I believe my mom did the right thing and I fully support that she made this choice).

My sister came forward, claiming to have been blindsided because she had a perfectly fine relationship with Mom.

Thank goodness mom told so many people and kept those spiteful letters. Any lawsuit, no matter how clear the facts, is expensive and emotionally rough.

Be there for your mom. Write your sister off. My prediction is that some day, when your mom is gone, this anger and hate will be focused on you like it was on me. Get yourself outside her range.

I used to be aggravated listening to Mom grieve over her relationship with my sister. My children are aggravated because they have heard me do the same. Now I know that mom was 100% right.

I’m not sure what your sister has to do with where you live. Your mom sounds a bit dependent. Perhaps it’s time for her to look for an assisted living facility, especially if she has dementia. Having said that, I can’t imagine moving for a 10 or 15 min diff in a commute. It’s all relative I suppose.

I totally understand where your Mom is coming from, however understanding and agreeing are two very different things. In my opinion I think there are a few options. 1. You take the job and move. 2. You take the Job and both you and she move. (not necessarily together but close by.) 3. You don't take the job and regret it and resent her for years. No matter what happens you can not stop your life. You need to move forward in way that is comfortable for you. Also you might take her on a 25 minute drive to see how close that really is.

Maybe you and your mom can look for places that are nearby your new job. If that doesn't work. Maybe reconsider the drive as a positive. I used to work 21 miles from home and I used the drive as a buffer between work and home life. But if you hate driving that may not be for you.

you have options but the emotion is making it complicated.....I understand and totally get it. My mom became very needy and even though her dementia wasn’t debilitating, it was also somewhat manipulative at times. My husband and I were living with mom and prayers to God for the best plan for mom and us were the only way we found peace and the best answer. Mom choose an assisted living which she could afford. I researched using which showed me prices and then I called and set up appointments all on 1 day. She chose the 1 bedroom (Atria Senior Living). It was a trial stay and after 2 months she began to call it her condo and now at 7 months she is content. I feel so relieved that she is safe, her meds are administered (because she cannot be trusted to remember), she had delicious meals, events, hair done every week and is checked medically monthly. I take her only to the dentist and eye doctor. We live close by and our relationship is good. I got a echo show from Alexa Amazon so I can drop in. Mom has good and not so good days but her independence and comfort is wonderful. I would suggest making a list of pro/con and pray over it until you get peace. God prepared mom’s heart because she was stubborn and scared and difficult. God prepared our hearts because we didn’t want to do anything that was not the best plan and to find a place for mom that is a win/win. Prayers for you as you seek God’s will. It is a good place to be.