My in-laws have never warmed to me because I'm just a different person to them. They are very insular and live in each other's pockets, they always have done. GMIL is very much the Matriarch with MIL being next in line. All is done to their requirements and whims.
I have a small family and we are spread around the place, in different countries and so I'm used to distance and not seeing my loved ones for longer periods of time, I find the constant need to talk, text, visit, gossip very claustrophobic. I am
also not down with this idea of changing my plans to suit another's whims.
My SIL, however, has a boyfriend who has been welcomed into the family with open arms because he very much fits in with the tight-knit type of family they are. He defers to MIL and SIL on everything.
Maybe that's just the case here, maybe the ex is just more "their sort of person" than you. I wouldn't take any of it personally, or force a relationship. Just carry on being polite, friendly, remembering birthdays etc - remember, you married your husband, not his sister and whilst it's nice to have a relationship with the in-laws it's not required to love them or have them love you.