If you know more vocab than what WK taught you so far, you already are better off than I was when I started reading チーズスイートホーム ^^ I had to ask a lot for help…
Breaking down sentences like was done for the first few volumes of the main series is tedious but I think it helped me a lot, give it a try if you have the time 
(the link to the treads are in the first post of the club thread if you want some template/inspiration).
It seems to me that you will need some context about the characters but I guess we can give it to you if you are ok with being spoiled.
Looking quickly at the first few pages, I would said that the story if this spinoff starts at the earliest between books 5 and 6 of the main series (based on (spoil of the main series up to the last two chapters of volume 5) “the black one” being part of the story while the family is in the 2nd home).