Ohhhh they are these awesome UK kids books -- three of them are boarding-school books, which was my entry point, but branching out into adventure books and family-centric books as well -- written by Antonia Forest from the 1940s to the early 1980s. (As a result, sometimes the later books are oddly anachronistic.) The Marlows are a family of six girls and two boys; the girls all go to a boarding school, Kingscote, and the first book (Autumn Term) follows the two youngest, who are twins, as they start their first year at Kingscote at age 12. (Nicola, one of the twins, is the main POV/sympathetic character for most of the books.) If you like boarding-school books along that age range, I very highly recommend these. There is also one of the books (Peter's Room) where they, along with their neighbor/friend Peter, participate in a role-playing game (although of course they don't call it that), modeled on the young Brontes, which I also loved. I turned up this article, which I think does a pretty good job of talking about the series.

There's a community on DW, trennels, which isn't particularly active (the LJ community it replaced was more active), but there are a lot of people on DW who love these books (and in fact it was one of my LJ/DW friends who got me to read them many years ago). It's often represented at Yuletide and so on.

The problem is, the books are out of print (I don't know where you are geographically; maybe they're in print in the UK? Definitely out of print in the US though) and hard to find. However! You can contact coughingbear who has access to electronic copies. (I'd offer to send you what they sent me, but I don't have Autumn Term or End of Term in e-copy as I managed to snag hard copies of those quite a few years ago, and you probably want to start with the former.)