I am not either sister in this scenario and none of this particularly bothers me - I'm more interested in the etiquette of the situation.
Sister 1 is getting married this weekend.
Sister 2 has just announced her first pregnancy today. She's also getting married in a few months' time. She hasn't had any issues conceiving or anything like that. In fact, as far as we knew, she didn't want children. So it's come out of the blue.
Sister 1 has children so that isn't the issue.
It's more like - is this a mean thing to do?
Should Sister 2 have waited until after Sister 1's wedding to announce her pregnancy? Is this a way of explaining why she won't be drinking at the wedding? Something else?