So my mom has dementia now and wants to go back to where she used to live. We've gotten into arguments about it. What do I do?

I don't see where its unusual for a stepfather to leave property to his own kids. It must have never been in OPs mothers name to be able to do this. Seems OPs Mom suffers from Dementia and has been a problem for stepkids so they told OP, biological child, that they could not care for her. So now Mom is living with OP.

I am guessing that StepDad didn't raise you? Nor did you live with his children as a family? That he owned the farm before he married ur Mom? Only way he could have left it to his kids. If Mom is on the deed, half the house is hers and she has a right to live there.

But the problem is her wanting to go back and she can't. And with the Dementia, she can't be reasoned with. All you can do is redirect her. Which is hard. Or tell her you'll see about it later, your busy now and pray she forgets. I would just go and get the rest of her stuff. Don't say anything. Store it where she doesn't see it.

Arguing with someone with dementia is as pointless as talking to the wall. Just don't do it.

I keep forgetting that with mother. She can't 'track' a conversation any more and I keep thinking she can.

I stay to visit only as long as I can stand it, then I leave. She doesn't usually even remember I've been there.

Your step-mom was living on the farm with your step-father until last June. He passed away and a few months later the property now belongs to your step-sisters and mom is out. This sounds a little fishy to me.
Both of your parents are step-parents to you? What about your step-sisters? Do they not think they have some responsibility for mom's care?
They now live in her home that she was in up until a few months ago and now it's theirs and she can go pound sand?
She might have dementia, but damn right you shouldn't go and get the rest of her stuff. You should go talk them and maybe a lawyer.

Beaton has given you solid advice. Please stop discussing or arguing with your mother. When dementia is present it becomes an endlessly looping conversation that doesn’t change or benefit either of you. Change the subject or leave the room each time she starts about moving. Be sure she’s had a good medical evaluation that includes asking if there are meds to help her be calmer

So that other responders know: the OP's mom lives with her, her husband and young adult son.

More questions: does your mom have a medical diagnosis of cognitive decline, memory loss, ALZ, etc? Has this behavior started "suddenly"? She could have an undiagnosed UTI. UTIs are extremely common in senior woman and often have no other symptoms than change in behaviors, confusion, cognition, etc. It can be accurately tested for and antibiotics can restore her health.

Has she ever created the PoA documents to designate someone as her medical/financial representative? This should be done first. If this doesn't happen the only other pathway to legally act in her own best interest is to pursue guardianship through the courts. This is time-consuming and can be expensive. If this doesn't happen, then the county will eventually need to attain guardianship. SOMEONE will have to have legal authority for her in order to make decisions on her behalf when she is no longer capable to do so.

To answer your question, "What do I do?" It depends on what you wish the outcome to be? If she doesn't have a UTI but DOES have cognitive decline, she won't recover, she'll only continue to get worse. Do you want to be her caregiver? Do you want to subject your husband and son to her disruptions and constant needs? Who will take care of her during the day while everyone is out of the home working or at school? Please remember that your husband and son and your own physical and mental health need to come first so that you are wholly able to take care of your mom. This may mean considering transitioning her to a facility. If she doesn't have the financial means, but she does qualify for AL or MC, she can apply for Medicaid (however many states Medicaid do not cover all or some of AL).

There are solutions. Have you discussed the impact of her living in your home with your husband and son? Your husband gets to cast the deciding vote. I wish you peace in your heart as you work through this challenge.

I'm confused...your step-sister "kicked out" your mom, but where is she currently living? In a facility? Or someplace else?