I think - having read other's stories - that is what has happened between me and my SIL. We used to get on really well - at least I thought we did. Then I got pregnant with DC2 when she was trying for her first DC; she had fertility issues and that's when she all but stopped talking to me. After a few m/c she had her first DC when my youngest was nearly 1 and (and bless her she has had 2 m/c while trying for no2) has just had another.
I was hoping - I'm 46 and she's 44 - that there may be some denouement in our relationship now she has what she obviously desperately wanted. But I don't think that's going to happen. Thing is she is my husband's sister - they are not and never have been particularly close - and have what I feel is a quite competitive relationship (which my MIL encouraged while they were growing up).
However I have to see her at family events and it breaks my heart at bit - but I put on my happy face and carry on!