If mother has dementia and you're the heir and other sibling trying to take your property that your father died and left what can you do?

As stated, there is not enough information, but this is based on a couple of guesses. Your father perhaps did not leave the house to you, he left it to his wife in the expectation that she would leave it to you. You need to know that she doesn't have to do that. If she is competent, she can leave it to whoever she wants.
You say your mother has dementia, and if so she is probably not competent now to make a new will. You need to make sure that someone appropriate (doctor, lawyer etc) has put in writing a statement that she is no longer legally competent and no new will be valid. You then need to find out what will was made while she was still competent, because that is what will take effect when she dies. If 'taking the property' means giving it to the other sibling now, rather than leaving it in a will, the statement that she is no longer competent needs to cover her lack of competence to make valuable gifts. It would be a good idea to get the lawyer to put a lien on the title of the house, so that it cannot get transferred without you knowing anything about it. It might be a good idea to get back to the lawyer who dealt with your father's estate, who is likely to have some background knowledge.

We really need more info to give you a good answer.

You say "your" father. Are these you Moms kids by a first marriage, half siblings? Was Mom ever on the deed? If you r his child and his will says the house is yours upon ur Moms passing, then they have no leg to stand on. You are his child, not them.

If dad left a valid will, then his estate is done as per terms of the will.

If no will, then he’s considered to have died intestate.
Heirs then have to be determined. His surviving spouse or her dpoa or guardian would need to file for “standing” as an heir just as you would as his child - I’d bet/hope there is some sort of priority for surviving spouse in standing order, but you need to speak with probate atty as to this. All his kids can file to be lineal heirs with their own standing.

if your excluded from his will or he died intestate & there’s 1st marriage lineal heirs who are not kumbaya with your mom or you, imo you need a probate atty who does litigation to represent your interests. Its subspecialty within probate attorneys and lots lots more costly.

I am getting that you will get the family home when Mom dies. It was left to you in his will? You have older siblings that were grown by the time you came along and never lived in the house. Thet r not talking to u.

How was the will written. Dad left it to Mom to live in but goes to you upon her death? Then there should be no problem right?

Darrin63. I'm not clear on the Details. Your Mom is alive and your Dad passed Away?

Could you possibly state your question in a clearer manner. It seems as though there are a few issues going on here but it is a little difficult to decipher what you are mainly asking?