Pros vs cons on this one... its good you are thinking deeply about it because once you tell you can't take it back, yaknow? if your SO is the one who realllllly wants the telling to be done, i'd advise against it. it's your life, your system, and your truth that is being exposed. If you are truly uncomfortable with the idea of it, then there is no good reason to share it now. Maybe in six months, two years, whatever you will want to share. There isn't a rush. Come to a consensus inside before moving forward though because if you do tell and then have a multiplicity crisis in the middle of telling, it could get into a sticky situation for everybody (we've been there). If you think they will be supportive and such, telling them wouldn't be a bad thing, but it's important to always prepare for the worst. If you did tell them and they reacted bad and it makes the system go into chaos now is not the right time. There are risks and benefits to telling so weigh it out. if you
do tell and there is loads of misinformation where you're living about having headmates, you should prepare some info to give them that isn't total crap.

that way they can learn what it's really like and multiplicity stops being this enigmatic thing.
the mother-in-law knows we are multiple and she's cool with it. At first she had questions cause she had seen some movies/read some books about it, cliche 80s junk, so her idea of DID was one thing when in reality its another thing. She wasn't mean but she asked lots of questions which were painful to answer, just being that honest was so freaky. Sometimes its annoying when its like she is playing "Guess who?" like trying to know who is out by what we say or how we act, which is fine but frustrating to feel like we're in a petri dish.

Overall it's nice though, she knows us individually, she does activities with us that each of us like, and we don't have to have all these worries like "if i say X does that conflict with what Linn said or Lola or Ell" and also it's no longer an awkward string of conversation at dinners about how "I" am a vegetarian/vegan/carnivore who is occasionally violently lactose intolerant but not now, so can i have that milkshake please? lol. But it could have went the other way, where the reaction is kinda "Oh the gods! burn the witch!" so you should think carefully about how you will feel if they take it in a bad way. If they do maybe they would change their mind in time and learning, so its not a final thing if it doesn't turn out at the start. With parents i'd approach it, if you are telling, in simple ways that people can understand. Like how in the morning you have to argue with X and with Y about whether to wear the dress or the pants.

hope this helps a little.
seeya, cleo