Hi there spudsy, & welcome to PC & the DD Forum.

What you say about your sister is a very possible scenario.

It is possible for a single alter to stay in for years, and memory gaps of many years are not uncommon.

15 alters would not be that unusual either, but it would be quite unusual for her to know she has 15 alters unless she has had a great deal of therapy
this would still not mean she is wrong, she may be absolutely correct, as it is possible that most of her alters are co-conscious & share their memories between them.

Please feel welcome to PM us with questions, because this is a complicated & much misunderstood subject, & we don't feel sure our answer won't have made you more confused

If you go to the top of the forum index & click on the tab called "disorders" you will find some definitions under dissociative disorders that may help you.