Letter of reason for invitation query

Hello,

I am a half Japanese-half American father in law living in Japan and I would like some advice on my query.

We've recently added a new member to our family and my American son in law has put me down as the inviting person and guarantor and I just want to know what to write in the letter of reason for invitation as per below. I'm new to this so please bear with me.

1) Purpose of invitation - To live in Japan and start a family with my daughter in law.

2) Background to invitation - STUCK HERE! PLEASE HELP!

3) Relationship with visa applicant - Son in law.

So there it is, any takers? If I made a mistake somewhere or I haven't added enough info please correct me on it.

Thanks in advance for your helpful advice and help

Ozawa