My [38F] family wishes to converge in Europe to meet my sister's new baby. However, only my parents have bothered to meet my adopted son [2M] and I am hurt on his behalf. I don't know whether to attend the gathering or not. Thoughts?

My sister has had her first baby and it has become important to everyone (my brother [45] and sister [35] and their spouses, and my parents) to travel to Europe where she lives to meet the baby as a family group. There has been a great deal of excitement about the new baby (including from me!).
However, I adopted my son at birth 2 years ago and only my parents have bothered to meet him (they love him madly!). In addition, my siblings did not send gifts or cards when he came home and they haven't acknowledged his birthdays.
Obviously I am hurt and offended on behalf of my son that there is a hurry to meet the baby together when they seem totally disinterested in him. I feel protective of him. To me, he is my beloved, adored child in every possible sense other than his genetic heritage. I cannot force my family to view him in the same way, but I find it unkind not to treat him as if he is family.
I am feeling reluctant to get on board with the family gathering. I feel as if by doing so I am endorsing their lesser treatment of my child. I don't really want him to be around people who seem to have no sense of attachment to him. On the other hand, our father has the early stages of Alzheimer's and it is likely to be one of the last times we are all with him while he is able to appreciate it.
How should I handle all of this in a way that both honors my son, and avoids being an ******* to my family?

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tl;dr: My family wants to get together as a group overseas to meet my sister's baby, but only 2 of them have met my adopted son in his 2 years. I am hurt, and confused about whether to attend the gathering.
Edit: I have received a lot of really wise and compassionate advice and I truly appreciate the help it's given me in thinking through this situation. Thank you all very much.


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