I'd back out of the wedding (tell her you'll attend if possible, but the flower girl gig is too much just in case and you don't want to back out at the last minute if baby is early or you're not up to going), and I'd tell your sis that you probably won't make her graduation, either - if you feel the need to, apologize to both, but say that it's just the timing, and newborn babies take precedent over adults no matter what!
A note to each of them around the time of their events saying how you're thinking about them and sorry you're missing out on the fun, etc. and maybe some flowers or something else they like would probably make them feel good, if you could swing it...I'm talking just a quicky, handwritten note, nothing major.
Heather, WAHM to DS (01/04) * DD (06/06). Wed to DH(09/97)