Its just seen as strange and turns people away from wanting to be a part of the family as well in the future. If your parents are making it obvious that they don’t consider them family then they will act like they’re not family. Its a two way street and many times people don’t understand that if you treat someone a certain way - don’t be offended if you get that same treatment right back.
Let’s say your mom meets up with you randomly for a lunch - not offensive at all
Your mom meeting up with her DIL for dinner - no problem there either
Your parents cooking dinner just for their 4 kids out of which 2 are married and ask that only their children attend - rude