Why are sister in laws so annoying!

My SIL is having her daughter's birthday party the same day as my Son's 1st Birthday party. She's known for weeks when I was throwing it and she decided since she's working the weekend of her Daughter's birthday she will just have it when I am.

What sucks is I would like to go and I want my nieces to be a part of my life. My sister is law and I are not close. My husband and his brother get along good but both work so much. Usually these birthdays and major holidays are their only time together.

I have a few longer distance relatives coming in and already coordinated drives so I can't just change my date. My SIL lives 45 minutes so we can't just pop over the day of the party.

She basically only cares about her family and it sucks because these are my son's only cousins.

I'm thinking about having my husband plan a day activity with his brother while she's at work.

My update :

I was feeling petty. I already knew she won't go for it. My husband is actually upset now.