Even ignoring all context, they're mad at you for not getting them a gift. That's not reasonable in the slightest.
They expect you to get a gift, yet don't think you're good enough for their shower? You're amazing for not feeling insulted or getting mad at that in the first place, and they have no right to be mad at you.
You don’t owe them anything (your SIL is incredibly rude and has broken all social contracts), but if you want to stir the pot, give them the loudest, most annoying toy you can when the baby’s born/ for every birthday.