It’s hard to keep your paws off of each other in general, but it might be a good idea to not make out right in front of their little sister or be too grabby in front of grandma. You can put people off with PDA, which is totally their problem and not yours, but it’s probably better to keep some distance the first time around. Giving the fam a visual preview of the stuff you guys do in private is not exactly the coolest way to win brownie points.
Now that we’ve made it seem like this is the biggest moment of your life, yes, we’re telling you to calm down. Because it’s really not such a make-or-break moment. You are likely a really awesome person to be around, otherwise your partner wouldn’t want you in their life! So relax and take it easy, even through the awkward silences. If their sisters don’t like you right away, they’ll warm up to you. And if everyone is legitimately a nightmare, you’ll just have great stories to complain to your friends about later.
It’s likely that your partner is a little nervous, too. So if they’re being a little weird or acting differently, just take it in stride. You can always bring it up later if something was really off or offensive, but don’t bring up any relationship drama at the dinner table the very first time you meet their people. Also, their parents and siblings are likely worried about making a great first impression on you, too, so cut people some slack. It’s like a first date — things will get less weird eventually.
It’s hard when you’re nervous and there’s an open bar at their cousin’s wedding, but try your best to practice safe drinking habits around their family so you don’t regret anything later. If not for them, do it for your peace of mind later on.