No formal invite to wedding

by kay
(MN)

Our daughter in laws brother is getting married, we assumed we would be invited and the wedding is being paid for by our DIL's parents. My DIL sent me an email telling me that we would be invited to the wedding and could we take our grandsons and 2 cousins home about 9 pm and watch them. I relied that we could. Well, the invitations have gone out and we were not on the guest list. My feeling is to just tell my son and DIL that we will pick the children up at 9 and not attend the wedding. It is not a sit down , it is going to be cocktails and heavy appetizers so it's not a question of 2 extra meals. We have known the DIL's family and brother for years, kids all went to school together and they were all invited to our daughter's wedding.

Is this proper etiquette that we were not sent an invite and just told we could attend? My DIL and her mother pride themselves on being etiquette experts.

What do you suggest we do? I definately will send a gift.