Very often you might hear someone call another person a "family friend," which means that they are not related to this individual, but that they have known them in a variety of circumstances related to their family in some way.
Often family friends are still just acquaintances to the rest of the family, but they are seen fairly regularly at family parties and events. They might be cherished, "honorary" members of the family, or they may be people that you just put up with because someone in the family (an aunt or uncle, for example) is friends with them.
The term family friend is tricky in that it could mean someone that you don't know very well at all, and yet the words involved ("family" and "friend") imply that the individual is someone near and dear to your heart. Family friends are individuals you've known a long time that may have been at family birthday parties, weddings, funerals, and even vacations.
The term "family friend" either applies to someone in a negative or positive way. If someone has spoken highly of the family friend, you might speak up and say, "Oh, I know them very well. They've been a family friend for a long time."
On the hand, "family friend" is also used in a negative way, like when someone else is sharing a bad quality about the person, or talking about something off-putting they did. People then use the term "family friend" to distance themselves from the person by saying, "Oh, I don't know them very well at all. They're just a family friend."