Yes. However, she MUST report her marriage and ALL children (step or bio) on the forms as per the instructions, REGARDLESS of whether any of them will be immigrating.
Omitting inconvenient information (like stepchildren via polygamy) is in violation of US immigration law.
By all means, have the MIL petition her now, and then 10 years from now (or whenever the visa bulletin is current), let the chips fall where they may with her marital status. She can immigrate without a polygamist husband who refuses to divorce her, but accuracy on the forms is critical.