@Hzky3 I was 15, and my two siblings were 13 and 10, when my mum (43) had her 4th child.
While I can't speak to the parent side of things, we did bear a lot of the childcare responsibilities as my father died less than 2years later.
From what my mum has said, it was difficult as you do forget a lot. But it was easy as she had so much support and was essentially looking after 1 baby.
I think the difficulty came when he was a bit older. She had 3 children essentially self-sufficient, and the one that required school runs and much more attention. Space and time became challenging. I don't think my mum would have coped if not for myself and siblings. Admittedly, we bore a lot of the pressure, and looking back we probably had needs neglected as, teenagers require a watchful eye as you know. But perhaps things would have been less challenging if my mum wasn't widowed.
That said, my siblings all have a great relationship with our youngest. If anything he's given us an insight into childcare. We all played, debated and hung out together routinely.
I do think though at times it must've been isolating for youngest sibling as, to some extent, he was essentially an only child. By 10 we were all busy adults.
Anyway, that's my 2cents. I don't know if it's a useful insight