I'd say do it now, strike while the iron is hot - if anything, it might help your cousin out a little bit if there are some parts of the family who aren't being so supportive, to know that he's not alone. I don't think it would be taking anything away from him, he's already come out, it's not like there's much else to it - you say it out loud and then you flip the channel and pop the kettle on, it doesn't have to be a big thing if you don't want it to be.
My boyfriend's younger brother came out as gay before my boyfriend did to their parents, that was an interesting situation
luckily they were fine with both!