Wow, this is something that could very well come up for my brother and I. My father passed away when I was 7. My mother remarried when I was 12. Our house was nicer, so my step-father moved in and they kept his house and rented it out for a few years. Then sold it. The deal has always been that my mother pays off the house and he handles his house. Well after he sold, he opened a car dealership and unfortunately, that flopped after a few years.
Anyway, if my mother were to pass before my step-father, my brother and I would absolutely, no chance, ask our step-father to move out. The house will go to my brother and I, not to his 2 daughters at all. We know this is the way it is willed out. We get along great with our step-father, which could factor into my feeling, but I also feel it is a little disrespectful to kick him out. He has been a major part of maintaining that house, and it is his home.
This may be worded wrong, which I don't mean to, there's just not an easy way to say it, but what if your mother did not pass away? The financial issues you discuss would fall on your own and your sisters shoulders to bear. Why not just handle it appropriately without possibly decreasing the joy in life that your step-father has left?