You might find some helpful info in these threads too 
If you want to leave early, just leave. Seriously no one else will care. Or they might be sad to see you go, but it isn’t going to ruin their lives! You can always come and pick the rest of the family up later, cos you will be sober. Tell your partner you’re worried, tell them you might need to leave early.
I was quite surprised at the number of people who leave things relatively early, I just never noticed before when I was busy getting shitfaced. I also didn’t realise how much more shitfaced than most other people I was getting…
If you end up having fun, stay and enjoy it! Make the most of seeing people, spending time with them, that you will remember it all, not saying and doing things you’ll later regret, no trying to work out what you might have said to who, etc.
Plan something to look forward to tomorrow morning fairly early so you have a reason not to stay too late and something to enjoy on top of the fact you will have no hangover!