I am the baby sibling of 3 girls. Our ages are very close. I will be 42 on Monday, middle sis is 43 and oldest sis is 44. All three of us were born within 33 months of each other.
And my mom was only 20 when I was born.
Anyway back to your question.
My 2 older sister are best friends, but not until they were grown. They are very very tight and I am a 3rd wheel, but not in a bad way. To be honest I just don't have as much in common as they do and it is okay with me. I get along with them both great and talk to both of them frequently, I am just not best friends. They travel together, spend almost every weekend together ect.
Growing up I was a very sick child with a lot of medical issues. I was at the doctor all the time and had several surgeries. I think my sisters just grew a very close bond, because I was getting the "attention" even though it was really out of my control. We had a 3 bedroom house and my two older sisters shared a room and I had my own. They liked and wanted it that way. When I was young I did feel completely left out and lashed out in anger at them sometimes. I do think my parents could of handle things MUCH different, but my mom was VERY YOUNG ( I can remember her 25th birthday) and did the best she could.
I never got along with my sisters growing up. Even though all 3 of us were in high school at the same time we didn't even hang around together, have any of the same friends ect. We were all totally different.
Once I had my oldest son Christopher that is when things changed. My sisters were the BEST Aunts to him and our relationship then changed for the postitve.
My kids are 20, 16 3/4 and 6 years old. My boys are not really close, but they do get along and do things together when my oldest is home. They call each other occasionally. Natalie is VERY close to both of her brothers. She talks to Christopher every single day on the phone and when he is home she is his constant companion. Natalie also is very close with Aaron. He plays with her, rides bikes, will take her places with him and his friends ect. I feel vey blessed that Natalie has older brothers that invest time in her. I was worried because of the huge age difference, but it has been much better than I could have ever imagined. I also figured Natalie would be like an only child but it is not that way at all. I really think she will always have a tight bond with her brothers because they will always be looking out for her.
I really think it is hard to tell how your kids are going to be with one another as they get older. I think a lot depends on personality, but also intrests and where they end up living. Some also has to do with the spouses that your siblings marry as well.