Help -- what to do about invites

So after a fight with FMIL, who of course had to cause a problem with the invites as well as every other vendor, we comprimised by deciding that SHE would stuff the envelopes for her guests and I would track them all. But I fear she will mess it up. She's already implied that they won't go out in time. She herself has called herself disorganized. And she's always late for everything.

So....do I by principal make her stuff them or should I just do it since I'm more organized and then I am assured they will go out on time?

I just know that if I do them myself she won't even show appreciation by saying thank you....

I can't decide what to do.