My sister has always had money problems and now the collection agencies are calling me. How did they get my #?

They looked it up or, more likely, she gave it to them. Ask them -- the collectors -- for their business name and address. Then ask them if they are aware of the Fair Debt Collection Practices Act. Tell them that they are in violation and that you are going to report them if they call you again.

She listed you as a reference. Tell them you haven't seen or spoken to her in 10 years and to stop calling you.

Tell them that she changed her number. Then give them someone else's number. Preferably someone you both know and you hate. Maybe one of her ex-boyfriends. Also try telling them she's on vacation out the country and you don't know when she'll get back. Then give them the number of a Mexican hotel. You can also try answer in a Chinese accent and pretend to take their order. Recommend the Peking Duck.

Odds are good that she used you as a reference, or, perhaps you cosigned for her. If the debt is old & she has been avoiding them, they may be referencing information from older material that cross references you.