If you avoid family get togethers you will feel - and be - increasingly sidelined. You'll not know about shared conversations, plans made, jokes shares, nor kept up with the news in everyone's lives.
When young children are in play, they will always be a dominant topic among families. Hell, we're only in families because they (our parents) had children. So it's inevitable there will be a focus on grandchildren and conversations about various stages of parenting. It's something they have all been through or are going through.
But that's not all there has to be. With my BIL it's a shared love of a genre of films; with my dad and my MIL it's birdwatching; all the adults are particularly competitive about Gin Rummy that follows all shared meals.
Finding other common ground that allows you to keep those strong bonds with your family is more likely to make you feel connected to your family than to pull apart and be The One Who Never Shows Up.
As for the Mother's Day post - I think you're reading in to it something that wasn't there. Mother's Day is about appreciating mothers. Your mum was congratulating your sisters on being good mums, acknowledging their roles as mothers. It's not for acknowledging all your offspring* Mothers. God knows they only get one day a year.
*I love that word. Blame Finding Nemo if you don't like it yourself. Mine are Offspring, we are Parental Units. Dude.