Having a sibling (or TEN siblings, for that matter) is no guarantee that you'll have a lick of help or support when it comes time to care for elderly parents. I am the youngest of 4 and I was always my mother's primary caregiver, dating back to my teens, when my father died. If my mother fell ill, I was the one who took care of her. The others had children and it was assumed I would do it. As the DECADES went by and my sister's children got older, she helped more. By the time my mother was dying, my sister and I were splitting the responsibility, which was good because I had a toddler to take care of.
My two brothers did nada. If my sister had somehow been erased from the equation, I would have been solely responsible for my mother's care. Well.......that's not entirely true, as my DH helped care for her. He was a better son to her than the ones she gave birth to.

Or if I had never been born (because I was a major surprise) then my sister would have had the entire burden. Our brothers were simply not going to take care of our mother. Ever.
In my MIL's case, she had no daughters. Two sons stayed within driving distance and took care of her. The eldest....and favorite

.....moved halfway across the country immediately after grad school and never looked back, even though he could have gotten a primo job right here in Texas. He never really helped with his mother's care, but was quick enough to criticize what the other two did.
I've seen families with tons of children, and it's almost never the case where they ALL help take care of the parents. I'd say the norm is that 1 or 2 pretty much do the bulk of it. And many times......it's ONE who does 95% of the work and the others take it for granted that the worker bee will happily do the grunt work.
And you know.....Sometimes the siblings really DON'T appreciate that younger sibling. My brother wasn't happy when I came along when he was 10 years old. He always gritched and moaned that I got too much attention as the baby of the family. He resented me from the day I was born and never got over it. Try being a toddler with a sibling who resents you THAT much. Try listening to a 50 y.o. man complain (yet again) that your parents took "the baby" along with them to run errands, etc. but left him and another sibling at home. Well, it was the 70s and people did that. They were teens and I was 5. My parents knew it was fine to leave teens at home, but not such a great idea to leave a 5 y.o. home, so they dragged me along wherever they went until I was old enough to leave at home. And it's not as if he WANTED to go with my parents.....He just resents that they took ME along. Nope, some siblings truly do not want an addition to the family. Finally, as an adult, I just had to accept that he would always resent me and that I could do nothing about it since I had done nothing to cause it except be born. So a sibling isn't always a friend.
Brothers didn't add that much to our lives. If my sister had not been born, I'd have been diddled in the sibling department. She can say the same. Ironically, even though we're close now, she thought me a horrible pest as a child because I was so much younger. And I was.

I am closer to the "boys" I went to school with K-12 than I was was, or will be, with my brothers. They are more like my brothers than anyone else.