Help pleaseee

So my younger sister is 13 and has had her period consistently for 10 months however her period is now 5 days late and I’m not sure whether it’s something to worry about or whether her body is just sorting itself out. I’ve read up about it and apparently it’s normal for girls’ periods to be inconsistent for the first two years, but it just feels a bit strange because hers have been normal since she started..

If anyone can provide any insight we’d be so grateful as I don’t know how to help or reinsure her..

Thanks xxx