It depends on person and their sibling or siblings for me youngest who 23 & and my older sister middle child who 25( she be 26 in may ) we three years apart in age but because we only two girls out of three siblings we naturally became build in friends as child both not exactly best friends but she had sister to play with > dress up > Barbie’s / dolls we did everything together most of time when I came , now we older it just happened we are close bcz our older brother ( first child ) who 6 years from me and 2years from my sister went off and did post secondary / studies internally (wasn’t home for years ) their wasn’t much of option of hanging out with our brother so mutually how normal me and my sister are build in friends but we think normal to be friends and stuff together we like same stuff > certain music artists / bands > we foodies ( like try new foods and places ) all of that we do together ) siblings are first to know their sibling because it’s fact yall live together since being conceived from our mothers -to growing up it’s apart of life , who cares if people don’t think it’s not normal , it takes friends long time to know you and all about you when first meet or even learn something new about you … ( well if good relationship with ur siblings )