How can I convince my mother and sister that my 84-year-old mother should stop trying to raise the 16-year-old boy she adopted when she was 71?

Time to step.. in thie teen needs to be around a younger family at this time that understands teens. It sounds like he is very unhappy where he is and she sounds like she has had it when you reach that age time to know when to let go... As long as you are taking the boy in & not sending him back to foster care so unfair to do this Could not do if it was her own child...Good Luck!

Get a clergy or councler to speak with your mother, Make sure she knows that this person is to help with what is best for all. Let her talk to the person and then do it as a group. If your 16 yr old speaks and she starts her complaining while with the 3rd party clergy or councler is able to evaluate the situstion then they can give there opinion without picking sides. They will speak for you and it will not be You against your Mom. At 16 your adopted brother can speak up and a judge can give you custody if you haven't yet spoke with the young man and ask if he would like to live with you. Then get a legal document from an attorney to accept custody of him till he is 18 and considered an Adult at which time he can legally make his own decisions. Good Luck and God Bless.

What do I need to expect from my MIL who is in 6-7 stage dementia and how long will she live like this?

If you and your husband are willing to do what you can for the 16 year old and have him live with you, more power to you. As Carol suggested, getting an outside agency involved, should be helpful. The agency should be "neutral" and have no feelings of spite and other emotional baggage. It seems to me that if you and your husband can provide a loving home for this teenager (and I presume he already knows you quite well), he stands a chance of successfully settling in with you than he might at a group home. If you really believe in your heart that you and your husband can help this young fellow, then pursue the idea with all your might because if you don't pursue the idea, you will probably always wonder "what if". Wishing you the best.

Wow..... my hats off to your mother who at 71 would take on a young child to raise.
She's obviously got a very good heart in spite of being a nag. If she's thinking of a group home, are you saying that the boy has special needs? Since you are willing to take him in, could you possibly start with a "helping her out by having him with you a little at a time and with you more a time goes by?"

Whew. What Adoption Agency allowed a 71 year old to adopt a three-year-old child ? She's now 84 and he's 16? Get him out of that home and allow him to live elsewhere, possibly with you. Social Services needs to become involved.

Could you get a Social Service agency involved? You sound like wonderful people to want to help this boy, and an 84-year-old woman is not likely to be the right parent for him. She also may have dementia issues going on, though not everyone that age does. This young boy needs stability. Please see if you can get some backing. Your mom sounds like she will give you trouble no matter what you do.
Carol