Your husband and sister in law are horrible, cruel and probably on the look-out for a shag. Does that help?
Are you going to stay with him until there is proof-positive he's cheating (he might just be hoping)? Are you going to be his 'lesser-woman', the one who is ok for looking after the children and doing the housework, and for sex if nothing else is available?
You are doing the 'pick-me dance' and if things don't change you might be doing it for the rest of your life. Telling yourself you're not good enough, fighting to be as attractive as you can to a man who was supposed to have committed to you 'for better, for worse'.
Oh, and you know he's a liar. You know he fancies her, but he says he doesn't.
If he's checked out of your relationship in his head, why is he still living with you? I'd be wary of believing he was committed to being your husband, after this behaviour, which seems from what you have said to have gone on for years.
So, that's all bad stuff. What's more positive?
You're looking your best. Focus on that. You look great and that's good for your confidence. Congratulate yourself.
Do you have children, property? Do you have an escape plan? Are you earning? Does he think you're so dependent on him that you have to put up with whatever he does? Can you change any of this, to your own advantage?
If he's having or looking for sex elsewhere, it isn't your fault, it's his.
No blame attaches to you. And it isn't the other woman/women's fault - he knows he's married and what his commitment should be.
Can you get some counselling to work on your self-esteem? Sounds like you're worn down by this and some therapy might give you a shift in perspective.
Good luck. You shouldn't have to feel bad. Give it up. They're in the wrong, not you.