I'm pretty sure if the guy owed you back rent, you can keep his stuff and sell it to recoup your losses. You'd need to check the laws in your area, tho.
Your sister has no legal claim on the property at all if she's an ex. You could - out of the kindness of your heart - let her come get the stuff. And absolutely do not drive it to her; besides the cost of gas, you're going to be out the time it would take to deliver it. Honestly, if it was me, I would tell her no and sell the stuff as soon as it is legally possible to do so.
No need to be rude or anything, just tell her that his lack of payment and other activities are going to directly effect YOUR bottom line (loss of rent, possible difficulties renting an apartment with a drug seller rep) and you need to do what you can to minimize the problems. And make sure to explain that it is completely within your rights legally to do so.
And I agree - don't help out family anymore like that. I've been taken advantage of before by family members, and I'll never put myself in that position again since it is pretty awful to deal with the fall-out when stuff goes wrong. Just because they are family, it doesn't mean you have to bend over backwards to make their screwups easier for them to deal with... especially at cost to you.