I'm so sorry to hear you are going through this

I haven't been in your shoes but my Dad was adopted and while I'm not sure about how him and his brother (who was also adopted) were accepted in comparison to his cousins as babies, I know as teens and adults there was no issue with the extended family.

There were times though when I was growing up that my Dad was referred to as "one of the adopted ones" and not so and so's son, when strangers to me were trying to place who I was. (They obviously knew my parents or grandparents).

It's unfortunate that the stigma is there still and I feel sad for your daughter (and you for that matter) that they are acting in this way.

I would talk to them about how you are feeling. It would be better than keeping it bottled up