What about asking your dad directly if you can see him outside of your sister's house? Invite him and your mom for brunch at a nearby restaurant? Why not tell your dad directly you want to see him, just not at your sister's house? Why not be honest with him and give him the option of seeing you? Perhaps my advise is colored by own situation (I just moved across the country to take care of my ill parents), but I just hate the thought of your parents living out the rest of their lives thinking you don't want to see them. I think they would be better off knowing the truth and devising a way to see you outside the house.