AITA for stopping my grandmother claiming 1mil from my father?

My father, his wife (my mother) and his MIL (my grandmother) bought a house together. My father took care of the house, my dying mother, his kids and his parents in law. When my mother died, the house was 50/50 owned by my father and his MIL. Half would’ve been worth 100-150k.

In 2017 my grandmother gave her share of the house to my father so she could get maximum pension benefits (you get less or none if you own property). Her share would’ve been worth 225k due to property prices increasing since the initial purchase. She said it was a gift for him being the caretaker of her dying daughter and themselves. There seemed to be a mutual agreement that no money needed to be exchanged. My father has all the paperwork saying he is the sole owner of the home

My father wanted to build a dream home. My grandfather was getting too old to walk so we all agreed it would benefit everyone to move my grandparents somewhere he could access public transport. I purchased an apartment worth 375k that we would house my grandparents rent free with me and my father covering home loan repayments, utilities and strata.

This year me and my father paid for the knock down rebuild and now the house is worth 2mil. My grandmother blindsided my father by asking for her share of the house back, which she claims is 1mil. Her daughters have said he could go to court for elder fraud and they had the power to foreclose selling of the house. They said family profits should be shared. He was upset because he felt like my grandmother was going back on her word which meant his dedication to the family meant nothing to them. We offered to give them my apartment worth 375k but they declined.

My father told me that the house’s profits were for me only. My grandmother said she was there for me growing up but not her other grandchildren so she’s going to pass down all that money to them and not me. My grandmother called me to accuse my father of hoarding all the money, that she still had ownership of the house, called me stupid for thinking otherwise and living in my apartment rent free wasn’t relevant.

Our lawyer calculated deductions from their asking price and said they would win nothing in court (not to mention be accused of pension fraud). We told that side of the family to exclusively talk to our lawyer. I do not think she has right to 1mil. Max 225k might be OK. AITA?

TLDR Grandmother gave her half of the house which was worth 225k to my father a decade ago. She did this as a gesture of good will for my father being the main caretaker of her dying daughter and the rest of the household. She also did this to get maximum pension benefits. We housed them somewhere more central rent free and paid for the rebuilding of the house. My father intended on passing any profits down to me. My grandmother says she has a claim on half the house’s worth of 1mil and intends on passing the money down to all her grandchildren but me when she dies. I do not think she has right to 1mil.