Cottontail as pets

Question
I'm raising 2 cottontails that I found outside in my backyard. I've raised them since they were only 6-10 days old. I'm guessing they're females, but I'm not sure. Now they are 6 months old, and they seem healthy. Even though they are siblings, they don't like each other and fight a lot. I don't know why they fight, because I read that cottontails aren't very territorial.  
Would it be right to let the bigger sister, Tito go, because she is very shy, but Brownie isn't. Brownie likes to be where the action is, so when Tito hides all alone, Brownie's with the family. It seems she's an outcast, and I feel bad for her. She's just so shy, I can't get her to really interact with us. I'm worried that if we let her go, Brownie will feel lonely in the cage when everyone leaves the house.
Also, I'm wondering what the signs of sexual maturity is in cottontails. Lately Brownie's been "marking her territory" on forbidden stuff, and running around me. Also, just recently she's begun biting me softly after nuzzling me. Is that like a rabbit love bite? I'm the primary caregiver of her. And she isn't growing fur under her neck like I thought females were supposed to grow. Also, will they get their period?

Answer
Hi,

It sounds like you have two males, not females. Males don't get along at all.

Unfortunately, you can't just let one go. You've domesticated them and letting one go will mean certain death. Perhaps contact a local shelter or zoo and they can help you with taking one? It is also illegal in the US just let any animal go.

Both of your animals have been sexually mature for about 2 months. As spring sets in, you'll see more fighting, more spraying, and more leg humping. Some of this can be alleviated by getting them fixed, but this doesn't always fix the problem at this point.

Female rabbits don't have an estrus cycle.

Good luck!