Yes, the Family Permit can also apply to direct relatives in the ascending line of the EEA-national or their spouse.
The family member (your wife's mother) needs to be
dependant however.
In terms of Financial dependence it means your MIL needs the financial support of the either you or your wife to meet their
essential needs rather than to have a certain income level.
Food for thought.
Although as you know the FP doesn't cost anything and has a quick decision time so worth making application if you can develop a strategy that stands up to scutiny.
Take a look here:-
https://www.gov.uk/government/public...y-permit-eun02