mother budgie attacking a younger female

Question
HI .. I have 2 pairs of budgies. One pair, I've had for 2 yrs. About 4 months ago, a friend gave me another pair. No problems, they all got along well. About 2 months ago, Female #1 and Male #1 mated, eggs hatched, chicks are now 3 weeks old. Suddenly, yesterday, the mother is turning vicious toward the younger female. Pair #2 has never shown any interest in the nest or the chicks. My cage is quite large, about 4 feet high by 4 feet in diameter.

I can't catch the 'victim' to isolate her from the aggressive mom. I don't know WHY she is doing this because as I said, the younger female has never shown any interest in the nest so she isn't bothering the chicks at all. The two males, meanwhile, are perching on the uppermost part of the cage, watching this fighting and looking quite confused!

The Mom watches the younger female like a hawk and the poor little victim is scared and losing feathers. Since I can't catch her ( or the mom) any ideas ?  

Answer
Hello Sally! Nesting females get VERY aggressive twords any other birds, including the father sometimes. She doesnt know that the female wont kill the chicks, because normally another female will go and kill all the babies and take the nest for herself. The mother wants her babies to be safe no matter what. Your only option is to catch the younger male and female. Use a fish net or a towel to catch her and the mate. When they other pair stops breeding, take out the nesting box and you may allow them to live together. But its very important to seperate them. Otherwise the mother could actually kill the other female. good luck!