Which in-laws are permissible to enter Japan?

My wife and our family wish to visit Japan. My wife has relatives in Japan and was born there.

Wife - and myself
Daughter - and her two children

These are all blood relatives to the person we will be visiting. I'm pretty sure those are allowed to travel to Japan, as of now.

But there are two others who are not blood relatives, that wish to go too. Myself. I guess I'd be the brother-in-law to the person we are visiting.

And my daughter has a fiancee/or common law husband. They are not married but have been together for 5 years.

Can myself, and the fiancee travel with my Wife and Daughter, to Japan, along with them? And whom can I confirm this with? There would be a lot of preparations to make this happen and I'd like to get a "for sure" answer.

Thanks

I was looking at this for someone last week. Because of the particular nature of your question, I would recommend you direct your question to your local Embassy or Consulate. Note - the person I was discussing this with also did that and were told "no", but that may have been because of how they asked their question. the reply I set to them (which includes some of the links you will need to check) is below.

A foreign national who is to acquire the status of residence of gTemporary Visitorh for the purpose of visiting a relative / an acquaintance (in a case of visiting an acquaintance, only a foreign national who have a relationship to him/her equivalent to a relative, or who is recognized his/her necessity to visit Japan.) .
https://www.mofa.go.jp/ca/fna/page22e_000921.html#section2

The linked FAQ on that page has similar information https://www.moj.go.jp/isa/content/001361129.pdf and that took me back to the MHLW site (it is in Japanese, but auto-translate works well enough.) https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/0000121431_00341.html
Fortunately they now have border control measures #29 FAQ in English https://www.mhlw.go.jp/content/03112200QA_EN.pdf
Hiding on page 25 of that is this line:
Entry on a short-term stay to visit relatives or friends is outside the scope of this measure because there is no receiving organization. However, new entries into Japan may be allowed if their circumstances fall into the following types of exceptional circumstances.

My understanding was you had to be a direct relative to the person you were visit.

So those extras would be a no. And your daughters would be a 50-50 chance.

I applied visa for our family on the second week of earlier this month: Me, hubby and 2 kids. My biological mother is a permanent resident in Japan and was recently diagnosed with heart disease. My situation was complex because I was adopted as a child and my name was changed. I called the Japan Consulate and booked an appointment to show them all the documents (proof of relationship between me and my biological mother) I have. They said I can apply for a visa which I did a week later. A few days after I picked up and thankfully, they gave us all a single entry visa. I suggest you talk to someone from the consulate and book an appointment. They're very helpful. Hope this helps