Don't have exact rec of the top of my head, but do you mind 3P with twins? Not exactly the same thing, but I have read a few recently.
Does 3P with stepbrother twins count?
If yes:
https://www.mangago.zone/read-manga/eunsoo_s_good_day/
Regular 3P with twins:
https://www.mangago.zone/read-manga/the_twins_and_me/
https://www.mangago.zone/read-manga/aisaretagari_no_gemini/
If you don't mind a little bit twincest, there is also:
https://www.mangago.zone/read-manga/ai_to_makoto_moriyo/
I have been working my way through a 3P+ list, and lots of twins come up, lol
TLDR; top link is the closest to the request.
Step brother BL please. Or you can give me brocest instead. Either is good. Just, anything with BL family related were something i had finished. I feel like i had read all of incest BL out there. But may be you guys can give me one, one's so underrated that maybe i had not read yet.
Thankyou.