It’s different for everyone, for sure. I never had to care for younger siblings/relatives but I worked as a preschool teaching assistant when I was 20 years old, and although looking after young kids felt nice and came easily to me, NOTHING about the experience sounded the “I wanna be a mom” bells. You know what did? Finding a partner I wanted to spend the rest of my life with. It suddenly felt “real” — like I could conceive of a concrete future alongside him with our children. When I was single and everything was just hypotheticals, I didn’t “want” to be a mom. Within a year of being married (so, 26ish), I’d say I knew.
Just one perspective. :) If I was giving advice to someone who asked, I’d say don’t rule it out until you find a partner you want to spend your life with. Then feel it out.