Please remember that Santa is a make believe game.
My 9 year old knows perfectly well that Santa doesn't exists. But he loves playing along with it, so we all go along with it.
Let your DD take the lead, but you can carry on pretending if you all want to. It's a lovely game we all play together. Adults take it too bloody seriously IMO!
I would have a talk with your 9 year old about carrying on the magic, but say to her, her sister wasn't ready, and she needs to help you keep the magic alive for her.
Then say to your 7 year old - that you forgot to say that sometimes some mysterious presents do appear, and you've never really worked out where they come from. Some people say those ones are form Santa (or something, you have a night to come up with something better!)
Basically leave it open for her to keep believing if she wants to.
When I was little, I knew Santa didn't exist when I was 5! I remember clearly. But I also remember being much older and getting caught up in the magic and thinking Santa was real after all.
Kids have a different relationship to fantasy than adults. If you 7 year old said "look mum, I'm an astronaut" you wouldn't fret about whether she really thought she was, would you?
So find a way to leave the door open to believing in Santa, so if she wants to, she can carry on.