I do not believe you can bring your mother in law on Adult dependent relative as the requirement states.? maybe someone can correct me if I am wrong.
E-ECDR.2.1. "The applicant must be the-
(a) parent aged 18 years or over;
(b) grandparent;
(c) brother or sister aged 18 years or over; or
(d) son or daughter aged 18 years or over of a person (“the sponsor”) who is in the UK."
Except if your wife can satisfy
E-ECDR.2.3. The sponsor must at the date of application be-
(a) aged 18 years or over; and
(b) (i) a British Citizen in the UK; or
(ii) present and settled in the UK; or
(iii) in the UK with refugee leave or humanitarian protection.
See guidance for ADR
https://www.gov.uk/guidance/immigration ... ly-members
Even if your wife can sponsor this then it is extremely hard route to proof.
Evidence required
https://www.gov.uk/guidance/immigration ... d-evidence
My opinions should not be constituted as an immigration or legal advice.