If you know another person that is not your biological family then they can be a friend. You share your life with friends (and family) and you care and like each other otherwise you will no longer be friends but enemies.
Friends are not family or brothers or sisters, but they could become family. Friends are important until a person marries, many a time friends introduce you to friends and people marry by friends of friends. Some people are in breeders. After people marry they have friends of the family who are usually also married.
Bad friends can ruin your life, while good friends can keep you moving in the right direction. They have a concept called well-adjusted. The more well-adjusted you are the easier it is to succeed in your life by managing the various social group.
While family and extended family are biologically connected, friends are not, they form the largest part of people one feels comfortable with in rest, relaxation and activities.
With friends, you share life, you go out together and you holiday together and you even can meet a friend of a friend and make them your girlfriend or boyfriend, this is natural and you should accept it as a natural part of life.
Very close friend might even become family, and you should welcome such and not be religious or misguided about morality. If you like the sister or brother of a friend, you can ask them to set you up and ask them if they are fine with your intent.
Friends are formed as growing up, local children who live in the area or through schools and these friends are the best, if you are super social be a central guru and maintain as many of these people as you can invite them often to gatherings. With age some people find it difficult to maintain friends but you must fight against declining friends. You should also decline from fighting with friends because losing friends are hard to replace.
When you have a group of friends, it is much easier to bring a new person into the group and with time and alcohol make them part of the group. With friends and other it is just a matter of opening your mouth and saying stuff, anything regardless if its good, bad, stupid or all three. You also must go to gatherings. I would say 1 night a week is a great way of maintaining friends.
Keeping a social group is probably done around the local bar, or the regular outing, where inhibitions can be medicalized using alcohol and new friends added. It is a great skill to be able to meet new people and slowly become friends or meet new people and become a sexual couple and you must be skilled at it.
When people get together all they can do is talk. So open your mouth and say stuff, anything. Hopefully after a time you no longer care and the new person becomes an old person.