To meddle is to interfere. You can meddle in someone else's affairs, and you can meddle with someone else's things. Either way, you’re messing with someone else’s stuff and she probably doesn’t like it.
Meddle started out in the late 1400’s meaning “to mingle,” then it became a pain when it began to be used to mean “interfere,” which is how we use it now. For a little while there it also meant to be, um, romantic with someone, but it’s not used that way today. Meddle is what little sisters do, always getting in the middle of your business.