I don’t understand why someone else ( a relative) would want a separate person from themselves to have children?

Because they want kids around but don’t want to be 100% responsible for them. Also, statistically, if your sibling has four kids, that’s genetically equivalent to you having two kids.

It's weird I know. As if they think my body and life is all about them.

Just their way of looking out for others, they think children will bring them joy and company and most importantly care in old age…

I don't understand your context. What?

Sharing the pain?🤔

Because they're bitter

Sounds like they’re trying to mind someone else’s business.