My mom and I were talking and she let it slip that there was a rumor about 20 years ago that my dad (who has been known to sleep around) has another daughter. He knows nothing about this.

Someone told my mom that the woman my dad was "seeing" had a daughter after her husband had already had a vasectomy. The husband found out she was pregnant and freaked out, I don't know if she convinced him it was his or if there was a test done. They have since gotten divorced.

This is all second had information, but how do you go about deciding what to do? Where would you even start?

If I open a can of worms, more than 1 family will be affected, but if I don't, I could have another sister out there?

Can of worms? You mother already suspect's that your daddy slept around, or knew for sure.
The other family gotten a divorce already. Seems like you could go for it..

How do you even approach it? Call her up and say, "hey, can I get a lock of you hair for a DNA test? I think you might be my half sister?

Well, that seems abit blunt, you wouldent tell a guy you like, hey I like you want to have kids and get married.. but yeah, call her up and ask if she would be interested in finding out if your related.

Get the facts before you go poking in peoples lives, like talk to your dad. He may have more information that you need to know. Its very possible this daughter knows nothing, and may have grown up knowing someone else as a father, and without some testing nobody knows for sure.

Disrupting lives for what may, or may not be true is not the best course of actions at this time. Opening up a can of worms on maybes, and what ifs is not wise, so get some facts first. Be sad to find out this other woman had another lover besides your dad wouldn't it? Now what? And what about all the other females dad slept with? Boy you could have a whole tribe to track down, test, and see who's what.

Can of worms all right! Get some facts. Give this a lot of thought.