If you are Catholic, tell your family that you love them, but it is a holy day of obligation, not a matter of wanting or not wanting to be with family. If you made a prior commitment and a lot of people are depending on you to sing at choir, for Pete’s sake, don’t leave them hanging and then lie about it later! That in no way is the ‘good Catholic’ thing to do. That alienates the whole choir from trusting you, and you have something else to confess. If you feel like there is no way to participate in that evening’s choir, talk to the choir director rather than no-showing.
If you feel like you need to do both (Christmas Eve and Christmas Day) Masses, they take precedence over family. You can’t make everyone happy every time, but Christ’s presence in the Mass takes precedence over all creation. Invite Granny to come hear you sing. What grandma wouldn’t love something like that? A celebration of Christmas starring her little dearie singing up front?
If your parents have a problem still, ask what the root of the matter is. If it’s being together on Christmas, again, let them know this is really important and you already promised to be there, and invite them to come here you. If it’s a matter of resentment against you being Catholic and no one else is, then there are deeper issues you need to address as a family, letting them know that this is your religion, that it’s not a matter of personal choice but God’s established Church on Earth and in Heaven, and attendance at Mass is an obligation.
It sounds like you’re in a difficult position, but stay strong and adhere to your Faith. I’m the only Catholic in my family, so I know it can cause a lot of stress. I had to explain to a very furious mother why she couldn’t receive Communion at my wedding, and why confessing my sins to her was not the same as confessing them to a priest. You have to learn your faith, stand up for it, and stick to it. It’s hard, but you can do it. If all else fails and you have to go along with your family, don’t be bitter or a grump while you’re with them. Be cheerful and love them. As far as how long Gramma’s got left, that’s up to God to decide. She’s made it eighty-nine years without croaking, she’s probably a tougher lady than your parents are giving her credit for. Ask her first (maybe apart from your parents) if she’d want to come to your church for your special event, that it’d be very special to you to have her there and see you sing. You might be surprised.
God bless you. Good luck.