Praise be to Allah.
What you must do is to divide the estate of your grandfather to all his heirs as commanded by Allah, may He be exalted, with each male getting the share of two females.
If your father paid any of his own money during the building of the house, attention should be paid to that when dividing the estate. The same applies if you paid anything of your own wealth when buying the other house.
This division of the estate is not only required to discharge the duty of your father; rather it is also required to discharge your duty too, because now you are living in what you do not possess and you are transgressing against the rights of others unlawfully. You are not entitled to anything of this house apart from your father's share of it.
The same may be said concerning the house in which your paternal aunt is living; she is not entitled to anything of it apart from her share of her father's estate.
To sum up: you have to work out your grandfather's estate and divide it among his heirs, who are your father and his four siblings. Beware of wrongdoing and consuming haraam wealth and living in an ill-gotten house, because that leads to grievous consequences and a bad end.
We ask Allah to protect you and help you to restore rights to the people who are entitled to them.
And Allah knows best.