Hello and welcome to PC,
I'm sorry to hear that your father in-law is adding a lot of marital distress to your life.
Have you considered or tried having an assistant come in to help your FIL? I helped my grandparents for a while, years ago, which did relieve a lot of the pressure that my parents felt. My grandmother's condition did deteriorate to a point where I couldn't take the emotional pressure anymore ~ and at that point, my parents hired a home health aide to take over part of her care. I don't regret that decision at all!
There are lots of agencies (in the US that I know of) to help families get through the physical and emotional stress of caring for elders. Personally, I volunteer at
Elder Network, as a friendly visitor. Someone who visits elderly who live alone, or with others (and gives those in the house a well-deserved break). We read, talk, play games, and just hang out together. It's a terrific resource for people. Families like yours, those who are feeling overwhelmed by responsibilities and it's damaging other relationships.
I understand your predicament very well ~ as I've been on both sides. It's tough! And that pressure always hurts the other relationships. Seriously consider and talk about this option with your husband, to relieve yourself from extra responsibilities & resentment, and work on re-connecting with your husband. Not only that, but your father in-law will also become a little happier too! They LOVE getting people to visit them because they want to. It helps them see that there are still some things in life that they enjoy doing, people that they enjoy talking with, and they can still go places with others ~ rather than feel imprisoned in a house (or whatever).
I wish your family the very best!!