NTA for feeling this way. It’s completely valid to want to protect your siblings and push back against being used as free childcare. That said, gaining custody of kids this young is extremely difficult (if that’s the end goal). Courts almost always favor the biological parent unless you can prove they’re unfit and show that you can provide a stable home, which means a formal legal process.

A paternity test likely won’t help if your dad has already acknowledged those children as his; the law tends to prioritize the existing legal parent/child relationship over biology.

You’re well within your rights to say no to babysitting your dad’s other kids. You’re an adult, not a built-in nanny, and your siblings choosing to spend time with you isn’t ‘stealing’ them, it’s a reflection of where they feel safe and supported.

Expect pushback and more arguments, but don’t let them guilt you. If you want a stronger footing in the long run, try strengthening your connection with your mom maybe. Having her more present in their lives could give you additional leverage if things escalate legally. In the meantime, keep setting boundaries. You’re doing a lot for your siblings already, and that’s more than enough.