What Devora and Lilka said (and Never, who seems to understand the adoption dilemma, which is rare in someone who isn't living it
).
Ok to be sad, for you and for your ds and for his siblings.
But (and this is a big but), there is nothing to be gained from wallowing. Live your life, and your ds's in the best way you know.
I feel sad that dd has siblings she will never know. And I feel sad that ds1 might have siblings, but none of us will ever know.
And sometimes I feel bad for ds2 as ds1 and dd are his only siblings, and they might one day find others to replace him.
Adoption brings up all sorts of dilemmas, and contradictions, and sadness. But also great joy. And it is the joy, and the positivity that we have to cling to, as much as we can.
Just as a matter of interest, are you thinking of adopting a second time?