I'm so sorry op.
Being a bereaved parent is the hardest thing in the world.
People expect you to be grateful if you have other children, they play down our losses and want us to 'get over it' because it makes them feel uncomfortable if we grieve.
There is no getting over it, the raw grief becomes less intense as we learn to live with it, but its always there. We are never 100% happy, every good thing is tinged with sadness that someone is missing.
I don't know how old your dc are, but from a relatively young age I spoke openly to my dc about their siblings, I would let them know I'm so happy to have them and I love them dearly, but I'm also a little sad because your siblings aren't here too.
My sons are adults now (I also have younger dc) and they write their siblings names in my mother's day card with wings beside them. And every 'occassion' we have, we also have angel cake too, that's our little tribute to remember them alongside all the celebrations of whatever.
Over time you may come up with little traditions to remember your child too.
Please do remember that you're on a path that lasts a lifetime, you don't have to hurry up and feel better, or hide your grief for the comfort of others, do what you need to do until you find your own way to move past the initial fog of grief and learn to manoeuvre your way in your new normal.
for you op.