first period

so here's the thing, my sister is in the fourth grade and lately she has been an emotional WRECK. she has not had any little elementary school stress she's actually excelling in school rn and our home life is very healthy and she's a happy kid. i know this because we are VERY close. she hasn't developed boobs yet. she has no pubic hair but she is very tall and she is very mature for her age and we think she's about to start her period but she shows little sign of puberty. she cried for 5 hours yesterday and couldn't tell us what was wrong because she didn't know. she literally cried until she threw up but said she wasn't feeling like she had a . she woke up this morning supper happy and healthy. so we have no idea what's going on. thing is, i actually started my period in 4th grade but omg i had boobies and lots of public  and underarm hair plus that discharge stuff since 2nd grade. can someone please help me diagnose my sister with something? i want to help her and my fam thinks it's her period but i'm pretty sure it's impossible for her to get her period before anything else puberty has to offer lol. but all females are different. what do y'all think?