I think KLS hit the nail on the head. I'd go one step further and insist she have a an ob/gyn exam and go on birth control. With the obvious lack of parental direction (other than when she was around you and your wife), she might not realize the remifications of lying and loosing your trust...and it's good to let her know she can't get away with it.
Are you a sucker? No, just a tender hearted, caring schmuck! Better that than a cold hearted, judgmental JW.