K.C.
How badly do you want to rekindle the friendship? If you want to stay close to her, I suggest one of two things:
1. Go to the communion
2. Tell her you have a previous commitment but schedule a specific day and time to meet up instead - maybe lunch just the two of you while the kids are at school
If you don't care about the friendship and don't want to go, skip it altogether. Just reply - no need to give a reason - saying "Thank you so much for thinking of us. I'm really sorry we can't be there but we hope it all goes well. Tell her congrats for us."