Don't take it personally. Obviously there's more to this story than you know. This is really your husband's issue, and if HE doesn't want to develop a relationship with his brother and his brother's wife, maybe there's a good reason.
I think if your SIL is not calling back or accepting invites, it's probably because she's being loyal to her man. Who knows what happend between your husband and his brother in the past. And who knows what BIL and SIL have said to him or about him or maybe you, in the past, let alone what they are saying or thinking now. Maybe they had a fight that was never resolved, maybe your husband doesn't like his brother's wife, and so on.. Your husband probably has a better clue about what's up and it seems he just doesn't want to be bothered with his brother or his wife.
Be thankful he's (dh) is not pushing for a warm and fuzzy relationship. He probably has good reasons. However, I could understand if you were upset that he's not sharing why he and his brother aren't so tight. Perhaps see if he'll shed some light on the subject so you won't be worried that it's all about you.
Whether what you find out is good or not so good, just keep in mind that your husband is probably handling the situation the best way possible. There is a saying.."familiarity breeds contempt." Just because you're all related, doesn't mean the relationship will be a fruitful one.
Just support your husband on where he's coming from, and not dwell so much on how others are acting.