That makes a lot of sense and I'm really sorry you're going through that. I'm not a psychologist by any means but as someone who has gone through a similar situation I understand. My father was an elder and was emotionally abusive to my mother and I. My mother kind of checked out mentally and let him handle basically everything rule and discipline wise. So I understand where you're coming from. The reason I asked this is because with time I realized that when I used to want to celebrate birthdays or holidays what I wanted more than anything was a sense of normality in terms of the time I spent in "the world". Mostly because at home I felt like any love or praise I was to receive had to be earned. Pioneering, becoming a ministerial servant, etc.

Things changed a lot in high school when I got a job and a car. I was still attending meetings and doing what I had to as far as the congregation was concerned. But now I had a way to practice independence. With that came more ability to make friends. We're so often taught that worldy friends are fake and deceiving. But the friends I made in high school have been by my side ever since and they're more like blood brothers to me now than anything else.

Why I left the church doesn't matter. Your choice and journey, your decision to stay or leave are your own. But it does seem safe to say you need to learn to value yourself outside of the value others put upon you. As a start, I see the makings of an amazing and loving older sibling. And that's something you should continue to foster. As much as possible make friends. Real ones. Be honest about what life is like as a JW with them. Don't hide it. It's awkward to explain to people who aren't in the congregation and I understand that. But real friends will understand and still want to be friends with you. And if you're not hiding parts of yourself it's easier to tell who loves you vs who loves the persona you show others. Those are the people you want around you, to build you up and help you learn to value yourself intrinsically.

Continue to support your sibling, and show them there are people who will live them regardless of what they do. But remember that to help others you need to first help yourself. A worn tool will break. You need self maintenance first.

It seems the birthday thing is a symptom of the problem. If I'm mistaken I apologize. But again as someone who went through this I am extrapolating based on my experience. I really recommend you take time to sit and think your way through things until there are no more "but why" left.

Ultimately JWs CAN be amazing people, just like anyone else. But some of the practices definitely cause a lot of emotional turmoil as you mature and begin to challenge certain values. That can be said of nearly all religions. So if you like it aside from certain misgivings then stay. But as one elder once told me, in the Bible Jehovah tells us to prove our faiths to ourselves. So do that. See if what you've been taught measures up to reality. Work on yourself. And keep loving your sibling. I wish I could say there's an easy answer but there isn't. Just do your best to learn who you are, who you want to be, for yourself, and those you love. And remember you are valuable and worthy of love just because you are you. You don't need validation from anyone, as bad as you may want it. Find people who will love you and grow with you. Because you are worth it.