I got my first period today right and my fraternal twin sister is jealous (idk why well I kinda understand I wanted it before I got it) and she is having cramps and says that would be weird if we got it on the same day

No way, that's literally so cool that you got your first period! And it's not weird at all if your sister gets hers around the same time, even on the same day. It's actually super common for people who live together or are really close to have their periods sync up, so for twins it's like an even stronger possibility.

Okay so, first periods are usually all over the place anyway. Yours might be light and then disappear for a bit, and hers might also start weird. It takes a while for your body to figure out its rhythm, so don't expect it to be like clockwork for a while. Everyone's body is different, so even if you're twins, your cycles can still be unique, but it's totally possible they'll be close by syncing up.

Your sister having cramps could mean her period is coming super soon, or it could just be regular cramps for other reasons. Sometimes a little bit of tummy ache can happen before big body changes start, so tell her to keep an eye out. And congrats on getting yours! That's a huge deal and you should totally celebrate.

You can always tell a grown-up if the cramps are really bothering her. And btw, if she wants to find out roughly when hers is coming, there's a quiz on this site: how close is my first period? — takes like a minute.