Dear Aners,
It wouldn't matter if it was a beautiful Silver Aston Martin out of a James Bond film, or Del and Rodney Trotters Yellow three wheeler van... or even exactly the same car as your sister has - your sister would have some kind of joy sucking put down to deliver.
She is madly jealous of you. You are enjoying a new car after 10 years and she doesn't want you to enjoy it. She is jealous and wants to make you feel that there's something wrong with it.
DONT LET HER.
Also. Say this in your head on repeat. YOU DO NOT HAVE TO ANSWER EVERY STUPID QUESTION SHE FIRES AT YOU... Just don't.
Each one is designed to make you explain "DH chose it" or apologise for it in some way so that she can feel superior to you.
That is a major personality defect on her part. Pity her for not being able to contain her jealousy.
Also your mum is wrong. It is NOT your job to take all the flak Dsis hurls at you and try to keep her calm. She wants to pick a fight with you.
But what you can do is ignore it.
Take my word for it (a stranger off the internet, who knows little about cars) its a bloody brand new Porche.. For God's sake. Im sure its fab. But what does that matter. If it wasn't your car. It will be something else, anything else she can latch on and you will get more upset at the illogical and unfair remarks. So you have to build a tough wall., Don't give her any information if you can help it.
Shrug, laugh merrily, or change the subject. have some one liners up your sleeve.
Say to yourself. You know what she is like, don't expect her to suddenly be warm and wonderful. Your sister is entertaining herself by trying to pick on you. That is really really sad if that is the best she can do. Don't let her. You are blessed in that you can see this is an awful way to behave and are not following her poor example.
Some have suggested that you tell your Mum you are leaving and just go... and that is an option ( I used to keep a set of car keys in my bag so I could just up and leave in a similar situation. I didn't in the end but just feeling like it was an option and I could do it - made it much easier to deal with)
As for being a buffer between the other two. DONT BE.. keep your lips tightly shut and let them get on with it.
Your priority is your DD, and whether she's happy. Your other family members can look after themselves. If they can't be kind and nice to each other and instead turn everything into a sniping battle - why do you have to put up with it.
I can't really advise what you should do, stay or go, only you know. What does your Mum do when all this happens? Is it a support to her to stay? Maybe.. but your DD is your priority here, not your Mum.
In stead of apologetically explaining and saying things designed not to set them off... try telling them straight that you don't like the way they talk to you and you hope everyone could start again and try to enjoy the evening. It's worth a shot.
I also thought you did well correcting her about calling you a bitch and pretending its a joke. I note that she did it twice more and you told her off again and presumably it stopped so that indicates to me that standing up to her calmly as you did, makes her drop it. Which is brillliant!.
I thought @CariadDarling also had a great reply.
I think it sounds like you are fed up of all this behaviour and that will help firm your resolve and tide you through. Best of luck..