A.,
I have a sister who I love, but for as long as I can remember has managed to split the family for one reason or the other depending on who she is angry with. She's got a mean streak and holds a grudge longer then anyone I know. She has made scenes that are unbelievable and even at my grandmother's funeral.
Still, we TRY to include her because she's family. It doesn't always work for everyone, but we try.
My question is how did your friend's sister-in-law find out she was expecting? Did she tell her brother first? Is she close to her brother?
If so, you could make it a couples shower, then the brother could keep his wife under control.
Bottom line, how many family members will be hurt or upset if one family member is excluded? But, if there is a fear the sister-in-law will ruin the shower, I guess she should be left out.
Hard decision.....Blessings....