Websites on missed periods?

My younger sister just came to me, worried saying she missed her period. She is pretty young and just started, so I told her that I thought this was pretty normal (she’s not sexually active so I know she’s not pregnant). As she’s still kind of worried something’s wrong, I’m looking for websites explaining missed periods to send to her, I found some O.K. ones, but none of them said anything about periods being irregular when you’re younger (I thought this was fairly common?) Thanks for your help!