https://www.gov.uk/british-national-overseas-bno-visa/your-family-members
A dependant can include your:
husband, wife, civil partner or unmarried partner
child or grandchild under 18
child 18 or older, born on or after 1 July 1997 (and their partner or child under 18)
parent, grandparent, brother, sister, son or daughter (18 or older) if they live with you and they’re dependent on you because of illness, disability or their age
When you apply, you and your family member will need to provide evidence of your relationship and that you normally live together.