Is it selfish that I don't help my sister-in-law with the children?

Your duty is to help take care of the baby, not to take it away. This cannot be said to be selfish. It's good to take care of the children, but it's, uh, hard to do without taking care of them. Seeing that you are still a child at your age, it's okay to take care of the child once in a while. If I really give it to you, I guess your brother and sister-in-law are not at ease. Don't worry about this kind of thing, it's not selfish at all, and selfishness won't want to come here and ask netizens. If you don't help take care of your brother and sister-in-law, you will understand. After all, taking care of children is no small matter. You have no experience. Just make it clear that you can politely refuse.