What you feel counts, and if you feel like you don't want to do the regular family birthday meal thing anymore. That is okay! You cannot make yourself/ds/dh unhappy to make your mum/sister/father/pil happy. Where will it lead? To more unhappiness.
We don't have to always want to be with our extended families, it is okay and NORMAL to not always want to spend spare social times with them. You have a new life, requiring a new support network and you have every right to do that. Your life, NOT THEIRS!
There is an assumption from 'society' that families should stick together, spend time together, sometimes even live in each others pockets. This does not mean it is right and healthy for you to do this, or anyone else. To me this model of family is outmoded. We should be looking at a more flexible arrangement. Where families stop treating each other in a way they would not do someone off the street or a close friend, to allow that some family members do not want this, to give them space if they want it...
I could write a book... ;)
You do what is okay for you, not them.
And no they won't stop moaning about it for a long time, but as long as you are HAPPY your ds is HAPPY and your dh is HAPPY that is all that counts. Not wobbling lips and what you used to do ie do birthdays.
Okay...now I'm going to duck the abuse flying my way