Upvote:1
Depends on nationality, you may have to apply for a standard visit visa for your mother in law.
Details: https://www.gov.uk/standard-visitor/apply-standard-visitor-visa
You can apply online and show up for appointment, usually in VFS. After taking biometrics, it can take an average of 7 weeks at the moment. You can apply for a priority visa, if you need it earlier. You or your wife can be the sponsor and provide an invitation letter.
Your mother in law can stay upto six months for a visit.