so..........are they still cousins?
legal action ideas:
Give custody of the kid to the 15 yr old horny boy (his grandparents will become guardians of the baby since he is a minor) A baby in the house is probably enough punishment and incentive for a 15-yr old boy to be more responsible.
Make the mother pay child & spousal support to the 15 yr old, but him being a minor, the grandparents will manage the money. Hopefully this will result in a college fund, but since you can't educate stupidity out of a person, he might just spend the money on a pimp-mobile when he is 18 (if his grandparents have not squandered the money on designer baby clothes).
Everybody looses. I presume it is too late for an abortion?