Of course it would not be wrong to say that.
Totally agree with fuzzywuzzy "I'd return the presents, ask them politely not to send gifts and if they want to send cards to include DC1 or they'll be returned as well."
Or, if they send money I would, I think, say, please can the relative send the money for the two kids and it will be split by us three ways, or not send money at all.
Plus agree with PlinkPlonkPlunk, do it now.
I'd probably say something like
DH sees ds one as totally part of our family, as do we all, because he is.
When DS1 is exuded, DS1 feels very unhappy and left out, which we are sure you will understand is not how dh and I would like our son to feel at anytime of year, especially at Christmas.
Because DS1 feels increasingly upset to be exuded from his own family by cards and gifts which leave him out we have decided all cards and gifts will be given and accepted to include all three of our children. This means we would need to exclude any cards or gifts that do not include all the children. We really do not wish to exclude any cards or gifts or people.
We feel sure they will understand.
Then stick to it.