I want to say this as gently as possible, so please try to take it that way. It's not all about you. Somebody is always going to have something going on in their lives when you have something going on. That's just life.
I think you might be expecting too much from them. There's no good reason why your extended family should be planning major events such as weddings and children around your schedule. Wouldn't you think it was absurd if they expected you to do the same? Of course ! Would you consider putting off your graduation so it doesn't interfere with their baby hoopla? That's just as crazy. Neither of you is obligated to the other. Try to appreciate that.
Celebrate your graduation day to the fullest ! Invite your friends, so if your extended family can't be there you'll still have people with you to share your day with. And if they are blessed with a baby, celebrate their joy too. You can do both. It doesn't have to be either/or.
I don't have any ideas on how to "not flip out" because I don't think your situation is all that bad. But, if you really think they owe you to plan their lives around you, I think you should seriously ask yourself why. I hope you can find some peace about this.