I will try and keep it short.
I have 3 brothers, 2 are half brothers (different mums) one of my sil is expecting a baby and other sil arranged a baby shower which she invited most of my female family and my 3rd sil.
My 3rd sil asked me why I couldn't make it. Truth is I knew nothing about it until she said something (she didn't go either but saw pics on fb) it totally went over my head, I was busy that day anyway and this sil never includes me in anything, I don't have to time to be bothered by any of it.
My younger brother however is totally offended on my behalf and wants to go in all guns blazing wanting to know why she is like this. I want him to leave it, like I said I just don't give a shit. Tbh I think it bothers her that I never mention anything. AIBU to tell him to lay off.