This is starting to remind me of that moment in one of those American Pie movies where Stiffler and the pie-fucking-dude get tricked into engaging in oral sex by a couple of chicks. Watch out, man. Next thing you know you'll be snogging the cousin's brother.
The situation was already a little weird. Without judging right or wrong, it's highly out of the ordinary to chat with some girl and have her suggest inviting her cousin along for the ride. There is always that argument that any guy hoping for a FFM threesome should be equally open to a MMF threesome.
Dude. You were offered a threesome by two young hot girls who are cousins. Why are you sitting on your computer on this website. GO FOR IT!!! I wud say, rent a hotel room for one night, with a private jacuzzi. Get some champagne. Rest will happen itself. Btw, can u give me the names and numbers of these girls so I can contact them for.....ummmm....a....ummm.....business meeting.