Read the offense people :
It happened when the woman was stopped for speeding, and because of a 10-year-old warrant for a dog law violation and the fact she failed to carry her driver's license with her, she was taken into custody. If that doesn't prove she is a flight risk, what does? For all you know, it might be another 10 years before she is brought to justice.
Wait for her sister? Are you nuts? It could be somebody with a bunch of gang members that will come and beat the officer's head in and spring the woman. You can't have that.
Calling another officer? Whoa!! Now you are crossing the line. The cop needs to feel like a manly man. Can't have another officer helping him haul in some skirt.
So he left the a 7 month old kid with a 15 yo. Old enough to babysit, right? What? You want him to wait another 25 minutes and miss the Midnight Donut and Coffee special at Dunkin Donuts? Priorities.