Probably going to become an uncle within next year and a half or so

Buy the kids the biggest coolest toys, like get them a 5 foot high hot wheels track, or a giant princess palace. Your sister can't say no and it will piss her off

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I thought my uncle was the coolest person because my mom never let me and my brother get game consoles growing up but he bought us each our own DSi and my mom was so baffled he would spend that much money on something that she didn't throw a fit or anything. If you get your niece/nephew a vidya console before any other family member gets a chance to you'll be regarded as the cool relative forever (unless your sister gives him an ipad straight out of the womb like is the case nowadays though)

Children enjoy being upside down more than they enjoy being right side up. Every article says not to shake a baby, but that's only because they love it so much. Shake the baby is a safe and legal game that everyone can enjoy.

Love can be purchased and the price is food that the baby isn't allowed to have.

Honey buns and honey nut cheerios both somehow have honey so babies can't have them. It's weird but true.

buy them toys

Introduce him to spaceflight and get him Christer Fuglesang's books

I take my job as an uncle as one of damage control, I show my nephew non-r-slurred things so that he isn't only exposed to his parents' r-sluration.