To me there is a much bigger issue at stake, and it is the one that Oubliette has alluded to. We all know that JWs do not do compromise. They refuse to be uncomfortable at the expense of the comfort of others. By giving in to their demands on this issue, it will teach them that they are right and that it okay for them to infringe upon the rights of others. They will come to expect this treatment in the future. I have seen this vile behavior again and again in my own family and it is nauseating.
Also, it is your daughter’s decision, but she is more likely to regret not inviting the cousin than inviting him and letting the others do what they want, even if it means walking out. Why? Because this is a special day and only logical that she would want to invite the people closest to her. Additionally, if the others walk out, it is their decision; it’s on them, not her. She has done her part by extending the invitation to all.
Additionally, neither she nor you should have to worry about having to run interference during the ceremony/reception to protect anyone. These are presumably all adults that know how to act like adults. If anyone feels uncomfortable at any time, they can leave.