Can't determine cycle; irregular periods?

Hello ladies! So I am 19 and doubtful about my periods. I get my period every month but I don't have a specific cycle. I have been logging my periods since 2011 and I've noticed that my cycle can last anywhere between 25 to 35 days with no pattern to predict the next date.

I was never worried about this but recently my sister was diagnosed with PCOS and I was told that getting your period every month does not mean that you have a regular cycle. Is this true? Any help would be appreciated ☺