Hello, everyone!
Manju, my pet cockatiel, is one year and a half old. She has always been the sweetest little thing, will happily perch onto strangers and sing to them. We never had problems with aggressive behavior before.

She absolutely loves my brother, whenever he enters the room she'll run towards him, climb on his shoulder and start making her little peeps. Recently, she has been getting a little bit aggressive with him. She will hiss if he doesn't pay attention to her and sometimes lunge at his face. He has played with her in ways that made her hormonal before, so my guess is that she might see him as a potential mate. He has stopped playing with her and I advised against him letting her on his shoulder.

We are not 100% sure that she is a female, but her overall habits and personality seem to match the female description quite well. I have seen her flirting with her veggies before and it also checks out with how female birds do it, although she has never laid eggs or attempted to build a nest before.

So, my concerns are, first, if this is really just hormonal behavior or if there could be anything else that could be happening, and second, what can we do to avoid this aggressive behavior and how can my brother interact with her in a way that doesn't cause any more of this? Should we keep them away from each other for a while?

Additional information to ease possible concerns:
Both me and my brother are adults. There are no children in the house to worry about getting bit.
We own no other pets.
Manju has been checked by a vet very recently and is in very good health
She spends 8 daily hours out of her cage, mostly with me, as her daytime playing space is by my desk.