Hi OP, I thought I’d jump in and tell a little bit about my experience - please note it is not meant as discouragement in any way, in your situation I would feel exactly the same.
I moved to Germany a few years back, similar city to Hamburg (which I absolutely love!!!), with a well-paid internship and no partner or kids at that point. Through my line of work and social connections, I ended up being part of a pretty wide group of international young families, so I feel like my observations might end up applying to you and your potential move too.
Germany has a great quality of life, and at the same time is an incredibly challenging place to settle in (culturally and socially), despite what we all felt and said - just like your sister-in-law - that there is so much potential to thrive there.
The winters are depressing. I’ve lived through many British winters and summers, after being raised in a sunny country, and thought I was prepared. I was not. It is colder (fine, like you said) but also much darker. In winter, the “daylight” hours are 8:30am-3pm. And there is not much light. In Hamburg, you are not looking at snowy winters. You are looking at nonstop rain and drizzle, windy rain due to it being on the water, and very high humidity. It is also an industrial city and the skyline reflects the pollution. I love Hamburg in good weather, but during the wet, dark months, it is not picturesque at all. When the temps go below 0º, German cities also turn into big black ice festivals, free-broken-bones for everyone.
Health insurance is compulsory. And expensive if you do not have an employer. As a freelancer or job-seeker, I was quoted 500€ a month several years ago, and that was without dependants. I quickly applied for a further study programme so I could half the rate as an EU student. Some poorer migrants choose to ignore this requirement - not recommended, especially with a child.
Childcare is cheap, or free depending on the state. There are also incredibly long and stressful waiting lists and processes if you live in a big city.
People can be very cold, and racist. Some of the kindest people I’ve met were Germans, but there is also a very strong cultural boundary that says “you are not one of us, and we will remind you of it every change we get”. The subliminal messaging is there. People do treat you differently after figuring out you’re not German, and that happens quickly after they hear you speak. Your child will be the child of foreigners - considered German, but their parents won’t always be 100% included in the lovely child-focused communities which I understand you would love to be a part of. It will be tough to achieve integration.
I spoke no German when I arrived, but I already had 5 other languages at native or fluent level, so was confident in my ability to learn a new one. It took almost 2 years for me to pass my B2 exam (granted covid happened in between), and another one until I felt comfortable holding longer conversations in German with my peers. The lack of German was pointed out regularly - in shops, government offices, neighbours…
It sounds like your partner is Polish or Romanian perhaps… I also have an Eastern European family background and know the importance of family ties and how strongly they feel about being with “their own”. While this is a lovely concept and opportunity to raise your child around, I strongly encourage you to aim at developing your own life, friendships and connections outside of the blood relatives. Once children are involved, the in-laws can become overbearing or overwhelming in a cultural context where you might be expected to join or accept their practices by default. Again, this can happen to any person in any country! I’m sure you are already aware of certain cultural practices but with both you and your partner being in a third foreign country, alone, I fear his circle of relatives and acquaintances of the same origin, could end up being what is expected of you to join and make a priority too.
To summarise the whole thing, research and make plans to integrate in Germany. If/when you move, take all the help you can get from his sister, etc, but start building your own support system that is based off the local culture, language, and people. There are many immigrants in Germany, connect with them and foster that sense of belonging. It is very important to your future child or children. To feel like their parents are at home wherever they live, be it Liverpool or Hamburg… Integration is key.
I have many friends in Germany who came with their partners, without a job or with small children and planned to learn German while being a SAHM and having relatives nearby. Honestly, not one of them has actually learnt German or integrated herself in the community. The husbands work mostly in English or their native languages, and they find it hard to even join a playgroup because they can’t share much with the other mums. They do have those relatives or friends around, but their world has become very small.
Integration is key. Realistic research about it is also key. Not what your partner’s sister says, but actually what happens on the ground. Few people survive on one salary, few people have more than a couple of weeks of sunny BBQs a year in Germany. Plan for integration, and you will be building a solid foundation for your family to feel at home and settle somewhere you can be happy.