M.M.
We have battled this and I don't know the 'proper way' to handle it but here is how we have approached it.
We have a nephew 13 years older than our son and a niece 4 years older.
Our son was the first grandson on one side of the family but he gets lost in the shuffle with both sets of grandparents because both sets focus on the older grandchildren.
It bothered my husband and I and we have had multiple discussions on how to handle it.
Our son hasn't acted out toward his cousins but his female cousin does act out toward him, hitting him, taking his toys and 'showing out' when the attention she gets is 'threatened' by our son's presence.
Anyway, we opted to not mention it to our siblings or parents. We opted to give our son the oppurtunity to know his aunts, uncles, cousins and grandparents with no prejudice and we try to make the visits as enjoyable as possible.
We chose this avenue because we didn't want to sound petty or jealous to our own parents or siblings by bringing the issue up or cause any tension by bringing attention to the issue.
We figured our son knows he is loved by us tremendously and his other family loves him too, the other grandchildren have just been around longer and in our case, live a lot closer to their grandparents-- so it's never going to be equal anyway.
I know this probably won't be a 'popular' response or stance, but taking the high road makes us feel better and you can't 'make' others treat your children a certain way, so it may be more frustrating and disheartening to you and your hubby to focus on it, and may damage your child's relationship with his grandparents and cousins and yours as well.
Good Luck.