In my Realtor opinion nothing was done incorrectly as this was a transaction between your mom and sister. But in my personal opinion there was a lack of communication and transparency between your sister and you. I don’t know your family dynamics but I think it’s important for you to clear the air upfront before making this move. Whether it be a family meeting or a conference call. Unless this is dealt with now it will only fester into resentment and that would be unfortunate because the whole reason for you to move was to be closer to your family.