Tara,
I have great confidence your mom will beat it! Enjoy the party...let it be the celebration it should be.
Unfortunately, the cancer will be there the day after and can be dealt with then. Your mom needs this celebration...and so do you. The day after (and the days after that) you can deal with the upcoming treatment decisions, etc. But having this celebration reminds you of all the good things (and absurd things) that life gives you...and that will sustain you as you and your mom fight this!
Just imagine sharing with her the absurd conversation you had with your cousin and how thick she pretended to be about bringing children...I'm sure that she will laugh and say "how typical of (insert name here), do you remember when....." That story will lead to another which leads to another and affirms what it means to be part of a family (and be married for better or worse).
In my family one of my husband's brothers is getting divorced and the kids are having a hard time. We've been looking at old pictures and telling stories to the kids about the time their dad flipped the canoe, or their mom burned spaghetti when we were all over for dinner...or the time they creamed someone with a baseball bat (accidently) when it fell out of a closet. We've also been looking at pictures of us all before kids and telling stories about stupid stuff we did when we were teenagers (the kids are 19, 17 and 11 so they appreciate hearing stories about when we were all kids).
Good luck to you and your family!
Karen