YANBU, unless they gave her £20k as a gift, and the other £20k was a loan? (With some legal paperwork and repayment plan?)
As things are, you are NU to think they have blatantly favoured your sister.
I am sure they love you both equally, (and didn't think they were doing anything wrong), but they haven't given you a chance to understand or challenge the logic behind their actions.
Sometimes one family member (eg you) might be expected to make a sacrifice for another family member (eg your sister) if it's a life and death situation, or if she has a very serious problem that requires family members to act together unselfishly. (But higher property prices in London doesn't sound like such a situation to me!)
As it is, (ie they have acted without any group discussion), we can't know their motives and whether or not you would have agreed with them if you knew all the facts behind their decision & long term plans.
It is very hard to be 100% equal, and some inequality is to be expected. But this is clearly SO VERY FAR from equal that I would be upset in your situation. (We are not talking about a couple of quid more. It's £20k more and DOUBLE the amount you got.)
As things stand, you either have to bring up the subject yourself retrospectively, (and appear grabby/jealous), or say nothing and let the issue fester on.
It's difficult for you & I feel that your parents are 100% at fault here, by keeping this inequality quiet, & by not being transparent in their dealings with you both.
Probably best to broach the subject with them soon, and get to the bottom of things? But proceed with caution as it's not worth falling out with them over this. (Family bonds are usually worth a lot more than £20k.)
Good luck!