Frequently Asked Questions - Green Card through Parents
It will be faster for her to petition and she can do so as soon as she wants. I would suggest she should apply when she moves to USA. If you wish, you can also start a petition for your brother. He can wait in more than one category.
Sure. The fact that he is in Canada is not an impediment.
As far as I know your PD cannot be reused because you lost it through becoming ineligible for the visa category.
Review Form I-130. You cannot stay in USA while or just because the case is pending, unless you have some visa/status that allows you to stay.