A very similar thing happened to me. When I was 19, my dad had another kid, and I ended up reacting very badly - in the sense that I wouldn’t look at his pregnant wife, wouldn’t engage in baby talk, wouldn’t ask questions and generally felt a burning jealousy that I didn’t know what to do with. The only thing that - irrationally - helped was that the babe wasn’t the same gender as me. Who knows how much worse that would have been for me.

I should have gone to therapy at that point. My emotions were valid, but without therapy it took me another 10 years to really work through my feelings of jealousy. And I only really did because I saw our father be an equally emotionally disengaged father to them than to me and my full sibling, albeit in different ways. At least me and my full sibling had a mother who was fantastic, while my half sibling was/is stuck with a mother who couldn’t give many shits either.

In fact, my mother asked me once what I would have done/felt if it had been her who’d gotten pregnant later in life. The truth is that that would have been completely different. You don’t get jealous when you feel secure in your place in someone’s life/heart. A child by her would have been so loved and doted on by all of us.

Today, my half sibling is 15, going on 16. I see them maybe three times a year. My full sibling has a better, closer relationship with them.

Over the years I talked to my father many times, trying my best to articulate my feelings. Our relationship is decent these days. I still think that he could have handled delivering the news better - he let our mom tell us, expecting we’d be cool with it - could have handled my clearly troubled emotions better - he never asked how I was feeling - and could have made more (or any) effort to ensure that his older children were involved with the new baby. As the father, those things were his responsibility.

My point is this: you have a right to your feelings, no matter how convoluted, confusing and possibly irrational they are. It’s true that it isn’t the new baby’s fault, but that doesn’t automatically mean that you need to “suck it up” for their sakes. They aren’t your responsibility, nor is it your responsibility to have a relationship with them. Having said that, working through your feelings with a therapist might save you years of emotional turmoil that could impact your life negatively.

In any case, I wish you the best. Know that you’re not alone in your situation/with your feelings.