I think you're right that Brianna's mom's return, and another shake up in her living circumstances, are likely to stress the little girl out. Moving stresses everyone, especially little kids. And I think you're probably exactly right about what form that stress will take - tantrums, potty regressions, and so on are common expressions of stress in children that age. She may have a few hours or a few days of "honeymoon" angelic behavior first, but she's going to get to the less pleasant stuff eventually. Fundamentally, however, her mom's return is a good thing (yes?), and this rocky transition is temporary.
What might help?
- Talk to her about her mom coming home. When will it happen? What will it be like? Where will she and her mother be living? Who else will be there?
- Expect this stuff and deal with it calmly. Talk to her mother about what she was like the first few days with you, not in a "your child was so awful!" way, but to let her know what your coping strategies looked like, so that she can make use of them too, and so that the behavior doesn't come out of the blue to her.
- Stay involved. Obviously, you're a very involved aunt, and you've stepped up for your niece at a very difficult time. Let her know that you'll still be around, and that she can rely on you.
- Ask her how she's doing, and listen to the answer. She's three, so she may not be all that good at saying what's up, but listen to what she does say, offer her lots of comfort when she seems upset.
- Make sure she eats and sleeps enough at regular intervals. Nothing adds to drama like a kid who needs a snack or a nap.