I'm so sorry about your situation.
I feel like my FI and I might be in this same situation soon with bridesmaids and groomsmen (we both have 3 total). I feel like one of each might back out when it comes closer to the time. We just made out guest list and started to stress about who and who not to invite. Then we got to the F*ck it stage, and decided that even if we invite everyone we know not everyone will come and those who do come will have a great time! If people really want to come then they will, and if not they are the ones missing out!
I can also relate to your cousin situation, except it was my older sister who got engage about 2 months after us and now wants to have her wedding before mine (this summer). I was super upset about it at the time but am getting over it, as I know our wedding will be about celebrating us!