Despite having worked with kids/young people, I'm not particularly 'good' with them. I don't do baby talk, I don't have a good sense of what is age-appropriate and I refuse to position myself as a figure of authority. In the end it just confuses the hell out of a child and makes everyone feel awkward. Some of the time. On the other hand, kids appreciate my honesty and I love to play games, so we can hang out. But when we play, I kind of do it for myself and don't respond to their needs necessarily - so we could get into an argument because I want the doll to sit in a different car or something stupid like that, they insist, I dig my heels in and it's just going nowhere -_-
But you don't have to be good with kids, you're not applying for a job as a nanny. Some people get a bit offended if you don't think their offspring are amazing and coo over them but most are not necessarily looking at you for that. They're looking at their kid - to see how they meet you and how they interact. Chances are, your sister just sees different parts of her world meeting and is hoping that you and the kid get along ok.
Leonard Cohen (Bird on a Wire): I have tried in my own way to be free
Mrs Hudson (BBC Sherlock): Sherlock! The mess you've made!