[matese]

I do hope you mean take her back to your moms house with the little female Chihuahua and not your brothers house with the male pooch.
If you take her back to your mom's watch your mom's dog does not try to fight with Freya since she is going into heat. I had a pack of 3 females, 2 were not spayed one was, I never had any issues with fights but they lived with each other. Just be careful and do not leave them alone with each other, always have them under supervision.