Totally agree with Mountainboo, if your dad is confiding in you, then it is obviously your business. I hope it turns out to be nothing though! Be there for your dad. Then again probably hard on you too.
Perhaps not the right way but if it was me, I would wait and see what else turned up now that you are aware of it, be patient and then confront or ask her when you are almost certain.
If it is a family member, bits & pieces will fall into place with what other family tell you and you will be able to work out what is going on. Also if it is family chances are you have a good idea who, so rumbling them should be easier than if a stranger, whom you knew nothing about.