My DD is 14months (today) And she too is incredibly defiant - Stand up on the coffee table if I tell her to hop down she goes "onoooooooo" or "go awayyyyyyyyyy"

I think it is just the age, or pushing the boundaries and working out what they can and can't do... It would be hard for my DD as she see's the older kids doing things she's not allowed to... It's a hard life beeing a bubba...

I would say persistance is the key...