Will my sister-in-law receive half of my mother's estate when she passes?

Tell your mother to rest assured that your sister-in-law's power of attorney has nothing to do with inheriting property. In fact, it does not give a person any rights to property at all.

It sounds as if the type of POA you're describing is a general power of attorney for finances. Such documents usually only take effect if the principal becomes unable to make his or her own decisions. They're called "durable" because they endure in this way. It would allow your sister-in-law only to make decisions about and to manage your brother's finances if he becomes unable to do so.

Your mother is free to leave her property in whatever way she wishes"”by will, trust, or pay-on-death designation for some types of property. And if she doesn't already have some such plan in place, you might consider her to get one, given her concerns.