Mating behaviour in young cockatiel?

[Cinnanymph]

I have three cockatiels; Older Birb, Younger Birb and Baby Birb. Baby Birb and Younger Birb are direct biological sisters with a three year age difference and share a cage. Younger Birb was parent reared, Baby Birb was hand-reared and extensively handled as she had a poorly eye before coming to me (yes, she was checked by a vet and she's all good!).

Baby Birb is 8 months old, she is super cuddly and loves scritches. Everything was great until a couple of weeks ago when she started to display mating behaviour; holding her body horizontally and making mating squeaks. I thought this was bizarre as she was so young, but light Googling tells me that it's wiser to let cockatiels be 2 years old before breeding, but birds CAN show mating behaviour before this?

My other concern is that ... well, I don't want her displaying this behaviour to me :c She is not doing it in her cage like her sister Younger Birb sometimes does once in a blue moon; all times she has done it it is when sitting on my arm or shoulder. I only ever scratch her head, as I know with larger parrots you never scratch anywhere on their body to avoid the behaviour I'm seeing. I know she will be bonding with me, but I would rather she didn't see me as a mate, just a mom/caretaker. I take it there's no real way to discourage this behaviour?

EDIT: To clarify, all three birds are female. Baby and Younger Birb live together on one side of a divided cage, Older Birb on the other side. I've never had a male in the household and I am NEVER breeding my animals

 

[MayMaroa]

Don't encourage the behavior at all, walk away, let him know that we don't do that. Make sure he is getting enough sleep, try to give him more nights by covering w/blackout blankies. This might trick him into thinking its not breeding season. I

 

[MayMaroa]

Many birds when they first go to a new home. Where they get attention and are happy. Sometimes get carried away. It should pass. And you'll just be good friends. Don't reject her, and do distract and redirect, I've read a lot of behavior stuff, and that was the advice. It also shows she is happy in his new home. Of course don't pet the body, only the head. And stop anything's you know triggers this response.