C.T.
Aga:
Before deciding that her mood swings are hormonal, may I suggest two small research efforts. First, consider the time of day when your step daughter has her meltdowns. Is it possible that her blood sugar is crashing? As young girls enter their tweens, they often need some regulating of their blood sugar. A small protein rich snack (peanut butter, milk, cheese, yogurt, avocado) in between meals could help her cope with the changes in her body.
The second research effort would be to have a quiet talk with her where she does the talking and you listen to understand. Perhaps while you're riding in the car - just the 2 of you. Tell her how much you love her and ask her why she thinks she's been irritable lately. Then just listen. Help her realize it's okay to share by repeating what she says but not commenting on it. For example, if she says she's "just stressed because school is really overloading her right now." Respond with empathy and repeat her concern "Yes, I have noticed that you have a lot of homework lately. That must be hard." This conversation can set the stage for others and for some joint problem solving.
Good luck.
C.