ANSWER:

If he has proof that he invested the money or paid for it as a loan to his mother, he can keep it.

But if he gave that to his mother as a gift, he can’t take anything more than the inheritance which Allah has given him. 

QUESTION: Sheikh recently my grandmother passed away, and in her last will and testament she specified that the house she owned only be transferred to my father because he was the one who mostly paid for it and took care of her & that cost us much.However since my father has two sisters who are requesting a share of the property and since the house belongs to my father Is my father under any obligation now?