"Friends become our chosen family" is a well known quote used by many when talking about their close friends.
This quote really came to life for me when I came to college and met my forever friends. The friends I have in college will stick by me forever (especially my college friends and roommates).
Reaching unprecedented levels of comfort is definitely a sign that you and your friends have become family. You do not hold anything back anymore and you feel comfortable doing literally anything in front of them. These conversations you have are probably humiliating and oftentimes disgusting to anyone else, but they seem to be "normal" with your friends.
You willingly spend your money on them. You might start throwing parties for them, giving each other dinners and surprise celebrations. Maybe there is cake or your favorite candy waiting for you in your apartment when it is your birthday or "just because" you had a bad day. You do not just buy each other birthday gifts because you feel obligated, but you actually want to. You start buying them their favorite thing just because.
Your extended families start to intermingle. All of a sudden, you end up just as close with your friend's family as you are with yours. They take you out to dinner, they check up on you, buy you gifts for your birthday and holidays and call you their own child.
You print and frame your pictures you have together. And place them allllllllll over your home.
You check up on each other throughout the day. You text them to make sure they're still holding up after your intense group chat about your significant other or your least favorite co-worker. You probably get more "good morning how are you" texts from them than you do your significant other.
You introduce your best friends to strangers as "so and so who is basically my sister." They are basically just related to you at this point.
Everything is assumed to be together. Party invitations, clothes bought, meals cooked...you do not do ANYTHING without one another.
You're use to them being around so much, it just kind of feels normal or second nature. Long silences do not feel odd or awkward. Just sitting around watching TV on a Friday night is also pretty cool because you do not have to be "entertained." You just hang out, no other pretenses needed.
You realize that you're stuck together forever, and you really wouldn't want it any other way.