A tiny insight into sort of the other perspective.. although I haven't reacted like your brother and wife to be at all!
I'm the bride in this scenario, we are now on our third date for a wedding. We've had to delay trying to have a baby. My boyfriend's brother's wife is now pregnant and due two months before the wedding.
We had chosen to have a child free wedding due to numbers, location and feel of the day. We've had to change that as obviously don't want to exclude our future niece.
My future parents in law have barely mentioned our wedding, haven't offered to help, haven't asked any questions apart from commenting that the accomodation on site (which is totally optional) is expensive.
However, they can't stop talking about the first grandchild though, and how sweet she'll look at the wedding, what she'll wear etc.
The tiny bridezilla part of me feels a bit uncomfortable about this and sad that they don't seem to care about my boyfriend at all and baby, understandably, is far more important to them.
However like I said we're still inviting BIL and SIl because it would be bizarre not to! And obviously we want them there.