sink pipe in the attic is not vented thru the roof

What is the name of your state? Ohio

Daughter brought a house almost 2 years ago and we just found out that the sink pipe in the attic is not vented thru the roof. Can this cause health issues? Can they sue the house flipper that they got the house from?

it may be of a health concern but generally it will not cause a problem (but that doesn;t mean not to fix it)

2nd question: can you prove the seller was aware of the situation? If not, then you have no recourse. If you can, you may still be limited by the statutes of limitations. Maybe not though.

Probably nothing to worry about, this is a common practice down here. All of the vents in my house terminate in the attic; nothing makes it out the roof. Not as good as through the roof venting, but not usually a problem. The biggest problem is that you can’t as easily run a snake down the vent to clear pipes, but if you can get into the attic it is still possible.

To keep sewer gas out of the attic the vent pipe needs an air admittance valve (http://www.toolbase.org/Technology-Inventory/Plumbing/air-admittance-valves) on the end of it, they cost something like $15 to $40 depending on size.

If this is a new installation, something done by the flipper, you might call the building dept to find out if it is up to code in your area. If it’s older it probably was when it went in.