Does not have to be an ancestor - it could be ANY relative - a sibling of an ancestor - even someone from the in-laws family - or any in-laws family.
Could be the family member of anyone who married into your family. The spouses of your aunts, your uncles, your cousins, your nieces and nephews
Does not even need to be related to you.
I have used my spouses relatives and ancestors on the odd occasion - and they are HIS ancestors, not mine!! LOL
There is NO hard and fast rule.