You can apply for a UK residence card if you are a non EEA national and are a family member (spouse, fiancé, child, parent) or extended family member (brother,sister, aunt, uncle, nephew, niece, grandchild) of an EEA national.
You need to show that the EEA citizen is in UK, in order to work, study or live in UK as self sufficient in exercising their free movement rights.
You don’t need to apply for a UK Residence Card if you’re a family member but it can help you show employers you’re allowed to work here and help prove you qualify for certain benefits and services.
Extended family members must have a valid Residence card or EEA family permit.