Family member tenant

Possession - Section 8 and Section 21

My sister in law purchased a property and let it to her brother without any formal agreement (despite my advising her not to do so as she wanted to help her brother). Her brother works for her and therefore the arrangement was to deduct the mortgage payment from his salary. He later got married and subsequently had two children with his wife. The property is not a service tenancy as it has no connections to the business in any way.
Unfortunately the couple have now split up and he has had to move out so he is sofa surfing at the moment. What options are available to her for getting possession of the property to sell it with vacant possession?