Your husband cannot bring his mother in law just because he wants to.
He needs to demonstrate that she depends on him and/or she is part of his household. But still, recent changes made reduced the scope of Extended Family members who qualify (read bellow).
Application of these nature when there is no genuine dependency are likely to fail. In such case, your mum won't be able to apply for a visitor visa to the UK in the future as she would've shown an intention to settle in the UK.
I would like also to remind you an important change that happened in 2017:
https://assets.publishing.service.gov.u ... s-v7.0.pdf
Read this please (page 8 )