I think you need to halve their areas, and put in another doggy door. Keep them separate, except while you are right there with them, and stay on top of any problems. Spaying may help as that will remove the heat cycles and extra hormones during that time.
I can usually leave littermates together for several months, sometimes over a year. But especially when you are going to keep them intact, it is just safer to keep females apart unless you are right there, and they are not having any problems.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC