Is this "bribing" your children?

I often have to babysit my sister’s two children. They generally misbehave and don’t respect me as much as other older family members.
They seem to only behave exceptionally well when she promises to give them something in return. For example, if they behave well today, they get to see Spiderman in theatres on Friday. If they don’t, she will put it off (but will still take them).
Is this bribing your children? And have you ever done this to your own kids? What do you think about this practice?