Twenty five days is awesome!
With a 150 guests, is your absence really going to ruin your cousin's birthday? You have every right to say, "No thanks, I'm not feeling up to it." Oh sure, someone my give you grief for grief's sake. But I think you are placing too much importance on your involvement in this family affair.
Right now, your recovery is the most important thing you should be focusing on, not what you think other people will feel about your not being at a party.