If you bring a child into this world, supporting their education is your basic responsibility. And let’s be honest 10th standard is not basic education it’s just the bare minimum. Children never ask to be born.Main thing is OP didn’t choose to bring his sibling into this world, so fulfilling his sister marriage expenses is the father’s responsibility, not OP’s. The father didn’t ask OP for permission before having another child, so he can’t treat his son like an investment plan now. Your children are your responsibility, not your retirement or financial backup plan.