Is it normal for a 13 year old to skip their period?

my sister got her period a month before she turned ten and is now 13 since mom is always at work and with dad it’s a bit awkward so she told me that she skipped her period this month i told her it was normal for her age…am i right?
and no she is not sexually active i talked to her about it…
oh and what are the causes of girls skipping their periods?

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