Whatever mental help you're getting right now, if any, it doesn't seem to be working. Is there any chance of you finding a (different) psychiatrist to work with?

Fun Uncle (aka Funcle) is a difficult role to pull off. You can be "Rich Uncle" that just requires cash and no other personality. For the first few years, it's pretty affordable. The "quarter behind your ear" trick entertains and delights for a while before you need to come up with more expensive tricks. Or you could learn the disappearing cheesecake trick. Uncles seem to be doing it lately.

OP, depression is a serious condition and requires professional help. I hope you are getting that help. If not you need to… While the pregnancy of your sister is huge news for her - being an uncle or an aunt is a much lesser role and not necessarily something that’s going to change your life. Be yourself, you don’t have to be the fun uncle or the rich uncle.

At least you have self awareness, that's good.