How to politely turn down a job offer?

I recently interviewed for an IT position in Japan with A company, and they extended me a job offer with visa support. I initially agreed to the salary without too much thought, which is totally fault on my side.

However, after careful consideration to the salary offered (I'm a senior but I'm offered an entry salary) and my family situation (yesterday I learnt that one of my parents has some health complications so I'm afraid to leave the country), I would like to turn down their offer. It is also a really painful decision for me as working in Japan is my dream job, but I think that I think it may be not be the best time now.

How could I politely decline their offer in Japanse? I've only received the initial offer asking for the documents to start the visa process, but have not received the formal job offer and employment contract yet, so it's a very early stage (I only have information about the salary and location).

Thanks in advance.

PS: I currently don't have a job but already have very high paying offers lined up in my country, not relevant to my refusal but just adding it in case.

yesterday I learnt that one of my parents has some health complications so I'm afraid to leave the country), I would like to turn down their offer. It is also a really painful decision for me as working in Japan is my dream job, but I think that I think it may be not be the best time now.

This is quite a reasonable excuse. No one could have predicted it. If you feel you have nothing to lose between you and the company, why not be honest about this part? You don't have to mention all the other reasons. If you're lucky, they may be able to offer you an alternative.

Having recruited and employed many people over the years I recommend two things:
1: Advice them promptly that you cannot take up the position, and,
2: Be honest and tell them that due to unexpected family circumstances you cannot take a position in another country as you have obligations/ responsibilities in your country.
It could also be that when you do get the contract for the job that there are unsuitable things in there - that also happens, but this is before that step. I did have to advise someone to turn down a job because the employment terms were substantially different to what they were lead to believe (contractor vs employee as an example).