She's Tanuki's cousin but I don't know if they ever establish how far removed she is (like is her mom/dad Tanuki's Aunt/Uncle or from extended family).
Elaborating on this a bit, we had a very brief flashback in one chapter where it shows Tanuki in a sitting fetal position crying about something in a dark room before Komaru comes in (she looks like she's in middle school maybe) and talks to him. It was one of the memories Tanuki thought about when thinking fondly of how she used to be. It might be related to her mom/dad and Tanuki might have seen them be abusive or fighting but so far they haven't expanded on it.
There is that, and Japan also has something called "Added to the family registry*," which is being added to the Family like what happens with Adoption and Marriage, but it is also its own thing. If I'm right and that is what occurred here they are not blood related at all and are akin to the concept of family friends. Since it is not an adoption, the MC parents are not technically her parents but are her guardians.
Since it is a unique concept of their culture, it doesn't translate well either, so many use Cousin to translate the term for the title of that particular relationship when 2 are similar in age, and in the case of the parents, they would be her guardians.