Butt play is down at least for the sister. I wonder if Mizuki would be open to do that as well.
Anyway, their libido levels are insane especially the MC. Multiple chapters consist of MC and his sister banging each other brains out throughout the day, with them only taking a short break to eat or clean up. I wouldn't be surprised if the parents found them dead one day from exhaustion due to all the sex that they were doing, lol.