First, I'd like to offer my sympathies to you and your family, particularly your father and his siblings (but, of course, also to you and Lisa and your cousins), on the passing of your grandmother. I gathered from the conversations we've had over the years that at least you do have a lot of fond memories of her, although I'm never sure if that makes the separation easier or more difficult.
Second, well, I can, of course, not speak for everyone here, but given that relatives have joined the group in the past (siblings and cousins both), I, at least, can see no objection to having your cousins join us. Although.... it means it is ever more likely that we will need a third room if most of the people who attended in 2007 will be present again for 2008 - I know there were question marks over one or two of us (Slip, for example, given the proximity of next year's convention to his first wedding anniversary), and also that Glazer's brother Chris would likely be giving it a miss next year, but I was also under the impression that there was a greater chance of Angela, Kadie, and Simon (or some subset of those three) putting in appearances, which means the likely total number of people attending may be something like 17 or 18!
Still, if a third room can be secured (I have a high credit limit, so I can try my luck if people think it's a good idea, although Crax' comments about the odd state of bookings at the Radisson leads me to suspect I might not have much luck even were I to give it a bash), then it shouldn't be a problem for any reason I can see. But, again, I'm only speaking for myself here.
-Kain.