I have a similiar situation.
I own three budgies: Sid, Stella (same name as your budgie
) and Celeste. Sid and Stella are a couple, and Celeste is their daughter.
Stella became aggressive to Celeste and her sister shortly after they were weaned, she didn't recognize them as her daughters anymore. And I'm sure it had something to do with them being females, because Stella was instead kind with her son.
I gave Sid and Stella's son and other daughter away, and I kept Celeste. I hoped Stella would have learned to get along with her, but she never did.
So, I had to separate Celeste from her parents and now she has her own cage in another room, and she is really sweet with me and my relatives, and enjoys our company.
Of course, she still tries to reach the other two and interact with them when she has the chance, but Stella's aggressiveness is not going to stop, as just today she bit Celeste's toes.
Perhaps your Stella is like my Stella, she doesn't like to have other females around her, or around her mate.
If so, I would suggest you to keep Angel in a separate cage permanently, she will always miss her own kind, but if you can develop a bond and then spend enough time with her I think she will be fine.
I hope I helped you.