I've been reading this thread and I've decided to comment. There is little information in regard to your sister, the house, your FIL to go on and without knowing the exact nature of all your relationships.
1. Do you and your husband visit often/call him?
2. Does the SIL work and if not why? If she can, FIL should encourage her to do so. It could be the case that once he passes she may find it hard to manage with the cost of the house, bills. (depends on the size of the inheritance)
3. I don't think it matters whether she has three children by different men.
In regards to comments relating to she will be the one to look after your FIL. Sometimes this cannot be guaranteed, however this depends on your SIL nature and whether she is likely to look after him. Not sure why people think it is an automatic thing that she will be the one providing care. (I would like to think she would) In 10 years the SIL future may be different such as partner, spouse, job wise etc therefore it could be that she doesn't need it.
I would tell your husband to talk to your FIL about it and why he has come to that decision. Maybe think about the relationship you have with him. Is it close or is it strained? perhaps this could have affected the decision. Personally if your SIL and husband are both close to FIL and do visit often, help him it seems a bit unfair that the SIL gets it all by doing nothing (if she isn't working, you said your FIL is fit so surely doesn't need much help)
Your inheritance makes no difference in this matter. Your family could leave it to someone else, inheritance tax could take a huge chunk, you could divorce (with your husband likely to get 0.
To add - Your FIL is entitled to do whatever he wants with his inheritance and house.