Buy to Let Mortgages that are taken purely on a commercial investment basis are not regulated by the Financial Conduct Authority (FCA) because the property is not occupied by the immediate family of the borrower.
A Regulated Buy to Let Mortgage however, is used in cases where a property is already, or will in the future be occupied by an immediate family member. The definition of a family member is generally Grandparent, Parent, Sibling, Child.
Any extended family members are not classed the same and a conventional Buy to Let Mortgage should be fine. For example, if you are buying property to rent to your child, this will fall into the regulated category, whereas buy property for your cousin to occupy and rent should not.
There may be exceptions to this approach if, for example your family member resides in less than 40% of the property. This would generally be the situation if the property is an HMO where there are multiple other non-related occupiers.