Would U date your close cousin's Ex-Boyfriend?

It was a fling and i couldn't continue the realtionship bc it was causing too much drama so while we were friends he tried to talk to my cousin on myspace and she said she wouldn't date an ex-boyfriend of mine, but i seen the message without her knowin, when i was out of state she asked me if i mind them chillin but she has alot of guy friends so i said i didn't think anything of it and i trusted her so i didn't care, i didn't kno that that meant within the next 2 weeks they were going out ( officially bf and gf)and sleeping together and i was so shocked when she told me, the only reason i stop bein cool with him was bc i seen that message and was hurt bc after awhile i knew the drama would die down and i would want 2 be with him again, i told her how i felt and she felt bad but still is with him now and we aren't so close anymore.