Advice on meeting my Japanese In-laws

I'm going to be meeting my in-laws in a couple of months, my boyfriend says they're super chilled and non traditional at all, classic rock fans, former musicians and completely cool with foreigners but I'm still scared as hell cause my Japanese is still basic and I don't know what I could expect out of it. Im taking some traditional sweets from my country as a gift but I wonder if that's enough? Should I get presents for his siblings too?
Anyway, any advice would be greatly appreciated!

Just be polite, they will understand that your Japanese is not very good (but as soon as you utter one word they will tell you how good it is), so just do what you can and have your boyfriend translate :)
They might even try speaking in English to you.

About the gifts, instead of bringing something for each person, just get a big pack of sweets or snacks from your country that can be shared by the whole family, including your BF's siblings. Better if each piece is individually wrapped (as in a bag of assorted cookies in which each cookie has its own wrapping, or something like that). That's how you usually receive omiyage here.

If they are not very traditional as your boyfriend says, I don't think you'll have any problem. Good luck!

"Default status" of Japanese is atheist, like Swedish. Most Japanese know there are other cultures in the world and we must get along with them. So just be polite and ask what is appropriate manner in Japan. Don't put your palms together, since we don't do this normally. Japanese don't shake hands, but most of Japanese know shake hands is quite normal in western countries, so we don't refuse it. Just nodding is more natural, but if you like, shake hands.

Thank you so much for your advice!
To be honest I'm not worried about feeling judged or something like that, I know they are good people and I'm sure im going to feel really comfortable around them but one of my concerns is more about etiquette like, should I introduce myself in a normal polite way or is there something in particular one is supposed to say when meeting their in-laws? I'm not really sure how to address them and I want to be as polite as possible
Thank you in advance!

Just be sure to wear ganguro when you meet them, defy their expectations.

You are supposedly not married to your "boyfriend", so they are not your in-laws.

[Last name]-san will do fine.

Your BF should act as the go-between and he should introduce you/in-laws to each other. You should bow deeply (deeper than your in-laws) and introduce yourself in simple Japanese that you know.
For example:
Hajimemashite (はじめまして) - How do you do.
Noname desu (ノーネームです) - My name is Noname (use your real name, of course).
Yoroshiku onegaishimasu (よろしくおねがいします) - Very pleased to meet you.

You can score extra points by pouring them drinks (using BOTH hands, father first) during dinner.

I think one of the biggest mistakes people make in this kind of situation (me included many moons ago!) is trying too hard!

You are not Japanese and they know it and don't expect you to act like one! If you make too much of an effort to try on Japanese customs, you'll only end up getting it wrong and embarrassing both yourself and your hosts!So be chill, polite, friendly, open and just yourself!

Talk about things that are not overly personal but of interest to them - in this case, music in particular! Mention your fave bands, or any concerts you might like to go to, etc, etc.

Also, pay attention to your 'physical' background: if you see photos of them on the wall, for example, of them playing, mention it and ask them what the photo is about - they'll appreciate it that you were observant enough to notice.

And if your mum-in-law is doing the cooking, make sure to tell her without being over-enthusiastic (as too much praise generally embarrasses Japanese people) how delicious her food is! A definite plus is to corner her in the kitchen and ask her to show you how she cooked something you really liked!

In any case, they sound like they are cool, laid-back people, so the less you fret and worry and just relax (easier said than done, though, I know!), the better it will go for all of you!

Remember that despite cultural differences, the rules in this kind of situation are generally pretty global!!!

It's easy to fall into the trap of worrying too much about Japanese etiquette because Japanese people make such a big deal of it (it's almost as other countries didn't also have manners and customs!), but you aren't Japanese so they probably won't be too strict on you. Be friendly and ask questions, bring a small gift, take off your shoes when going in, and follow your boyfriend's lead and you should probably be fine.

You may find that your in-laws tend to speak very critically of their family members including your BF; this is one custom that surprised me as we would definitely not say those kinds of things to a guest in the U.S., but in Japan it's a way of showing humility and not seeming like you're bragging about the people closest to you.

Just so you know the context: meeting 'the parents' is a big step in Japan in your relation with your BF, where I'm from, it is something that happens easily, but when I met my in-laws it was clear that people expected marriage on the horizon. :-)
Don't try too hard, bring some nice omiyage, and most importantly, talk it through with your BF, he knows what his parents like to see best.