Her bet doesn't really work though? The original her said she doesn't have anything else she cares about, and that was true at that time.
If the bet were that would eventually change, sure, but to say she doesn't care about anything but her mother, when (to prove her wrong) you have to get her to; lose her memories, have her lived another life so her perspective is completely changed, and still have some information about her previous (now current) life so that she could have an outside perspective on it with her new world view...
It's not just an 'unfair bet', it's outright stupid.
It's like saying
"I bet if you would have different likes and tastes if you were a completely different person"... well no duh!
All this said, the witch might be a nice person who was just trying to get MC to realise and appreciate what she actually had - and what the witch, herself, doesn't have - a family.
There might be no real consequence to losing this bet, despite the fairytale.