On my sister’s birthday, my cousin announced in front of everyone that she was pregnant. So it was a really weird situation, and I don’t know how my sister will react to this. Has anything like this ever happened to you?
the same thing happened to me, at a friend’s wedding my cousin announced her pregnancy. everyone got pretty tense but nobody mentioned it to her face, later we gossiped a lot behind her back.
Does my cousin always do things like this abruptly, or was there a special reason? It feels like something’s broken.
@kahvebitmeden isn’t actually doing this for the first time. The tension between them has been there for a while; she might have gotten annoyed about something related to my sister’s marriage 
@soncaykaldi bro, this “talking behind someone’s back” thing seems really silly to me. If that’s the case, just say it and relax, what’s the point of getting upset? I don’t think cousin is at fault; it’s a birthday, but if it’s happening within the family, don’t make a big deal out of it.
In family relationships, these kinds of situations are always talked about, but let me say this: if the celebration center belongs to the community rather than to an individual (for example, if there’s a table set up and it’s not just about the cake), sometimes it doesn’t cause as much trouble. But if you’re assigning a special meaning to it, then that becomes a sensitive issue.
to be honest, it’s unnecessary movement. couldn’t it have been another day?
I think you should talk to your sister and reassure her; also check on your cousin’s motivation. This work will take time, we need to be transparent 
Does it seem like your cousin made this statement to overshadow her birthday? If so, the issue may not just be about the pregnancy announcement, but could also be an extension of the tension between them. Such things often arise from pent-up confrontations.