My sibling.
My sibling, I'll just call them Jupiter, Said a few days ago what their alters might be, and that they've been thinking they may be plural for a while. The day after that they were acting differently and said they don't know. The day after that they said acted a bit more different and said they they don't think they're plural anymore.
This seems exactly how my and my alter's struggle was with thinking if we/they/I am plural or not, but I don't know.
To add, Jupiter has been diagnosed has Bipolar, so that might be what the mood and personality shifts are.
Figuring out your identity at any stage can be difficult and confusing. The best advice we can give you is to give them space, give them support, and give them good resources. If they are plural, then great! If they find out they aren't, that's more than fine too! There is no shame in them exploring either way. Here's a good resource about general info, maybe it could help. Ultimately, they are the expert on them, above all else. Just you being there to talk to them is more helpful than you know, I'm sure.
Good luck to them and you.
- Adam P. [Sprite]