Is it obligatory for the questioner to give his sisters their share of their mother's inheritance, knowing that he paid a large portion of the price of the house that his mother owned in partnership with his father?

Depriving sisters of inheritance is not permissible; it is a great injustice. Your assistance to your parents in paying for the house does not make it your property. If you paid with the intention of recovering the amount, you have the right to claim what you paid, after proving it and swearing an oath regarding your intention. However, if you paid as a donation, you have no right of recovery, and that does not grant you a greater share in your mother's estate than what you are lawfully entitled to. The resolution of matters concerning inheritance and joint rights is to be referred to the competent Sharia authorities, not to a mufti who only hears from one party.