I think having your MIL visit on a B2 Visa is an excellent idea. She will have a taste of life in the US. Honestly, I have seen so many cases where the US citizen child sponsors a in elderly parent without fully considering if the parent truly wants to live here. It is hard for an older person to adjust to a new life. No extended family or friends.
Additionally, you should research cost of healthcare in your state for an elderly LPR. This may also affect your decision. Good luck!