I mean it’s highly doubtful a group of adults expected a 5 year old to be fully responsible for a one year old baby. This is just a thought, but maybe you yourself thought that’s what they expected of you, so you took on these responsibilities when you didn’t have to,and when your family saw you were doing these things, they figured you were just being a good older cousin and taking care of her.

Maybe you should look at it a different way, because like I said you were far too young to be actually “babysitting”. Everything is what you make it right. So maybe instead of looking at it like a chore, you can look at it as building a relationship with a lifelong friend. Those are hard to come by, you guys are so close in age as well. It may not seem like it now, but when she’s 18 and you’re 23 you will feel a lot different about that.

You’ve mentioned in other comments that you have a hard time saying no, you live at home and you have social anxiety to the point where suggesting therapy isn’t even an option because you can’t even leave your house to get there.

Not leaving much room to help yourself here so I’m not too sure what kind of advice you’re looking for here when you’re turning down any suggestion that’s been thrown your way. So I think changing your thought process on this whole thing is your only way through this.

It’s hard to say if you’re the one in the wrong here, when considering everything in the post and all the comments, it’s leading me to believe we’re missing something to the story. I’m not trying to say you’re being dishonest or judging you it’s just something isn’t really adding up here.