Am properly gutted about this.
DH and I got together when DS1 was just a few months old - we'd been friends for years before that. DH's family were very welcoming - his mother was a single parent back in the 70s (DH and BIL have different dads and FIL is their stepfather, no kids with MIL), and went overboard buying DS presents etc. He was always included. His bio father isn't around - DH is daddy, MIL and FIL are granny and grandpa. DH considers DS1 his and will say he has 3 kids without hesitation.
DS1 is now 10. DH and I subsequently had two more DCs who are much younger (4 and 3). We noticed that DH's extended family (his uncle, his grandma), treated the younger DCs differently. For example, they send the younger DCs xmas and birthday cards but not DS1. Last year, DH's grandma sent the younger DCs xmas presents and left DS1 out. Our family xmas card was 'to Mr and Mrs DH (not my surname), DC2 and 3' - no mention of DS1.
Now that really upset me, actually, but I made allowances - DH's grandma is elderly and I'm sure things were different In Her Day. We see DH's uncle rarely enough that "my first great-nephew" comments can go overlooked. If DS1 has noticed that his siblings get much better/more presents from these people than he does he hasn't said anything (bless him).
MIL and FIL have always, I thought, made an effort to treat them equally. But things did change when the younger DCs were born. DC2 looks a lot like DH did when he was a baby and MIL made a huge deal about that to her long-term friends. I can understand that and it was an uncanny likeness.
MIL and FIL also told DS1 off quite a lot when he was being rambunctious around the younger ones when they were babies - but I could never be sure if it was unnecessarily so or I was being paranoid.
MIL works in Japan a lot and has made some friends there. A couple of these friends are in the UK at the moment and we went to meet them for lunch on Monday - they are staying with MIL and FIL down the road from us. MIL said (DH has some Japanese), that 'DH's oldest son looks just like he did when he was a baby', and DH interjected with 'no, my oldest son looks just like branston, DC2 looks like I did' - this caused some awkwardness as the guests had brought presents (as is the custom), but only for the family members they knew about. MIL was furious at DH for making her friend feel awkward, though that wasn't his intention and she hadn't told us she'd left DS1 out of her family.
DH and I talked about it later and he said that he'd previously heard MIL referring to DS1 as 'branston's child from a previous relationship' and 'my step-grandchild' - I feel so sad for DS1 because he considers himself to be their equal grandchild and, after all, has known them for much longer than the younger DCs have.
AIBU?