Isebelle, your SIL has had them for over 2 months so I think it's personally reasonable for you to request them back. Just say you want to use them again.
I have some DVDs out on a lengthy loan with a friend, but even though I miss them sometimes (
), I
know she's using them so it's okay. They're not my favorites anyway. However, your P90X set is expensive and special to you so an extended loan to someone isn't practical. Two months is plenty long enough to test drive the workouts and see results. If she never even tried them then that's her loss.
ETA - Let's face it folks: no one but other vidiots "gets" how important our collection is to us. The average borrower is pretty offhand and casual about it, and can't really comprehend their value to us.