I understand what you're saying.

I think cuddling and clingy are interesting terms to use when it comes to family though.

The important question is, what are you comfortable with? If you want to maintain that closeness with your sister, set that boundary with your girlfriend, ask if there's something you can do to help her be more comfortable with how close you are to your sister.

If you'd like to perhaps distance yourself from the cuddly/clinginess of your sister, you need to set that boundary with her and make it about you, not your girlfriend.

If your sister reacts poorly to that, that's her own issue and you would then need to sit down and have discussion with her about her inappropriate behavioural reaction to you trying to set boundaries with her.

Being able to set boundaries with people is important so while its hard it is good experience and a skill to work on no matter who it is you end up setting boundaries with, stick up for yourself and voice your opinions, what you want matters.