What does 骑在 someone's 头上 mean? I saw this on Jukuu.com:

我并不否认，你姐姐老是象个暴君似的骑在我们头上。
I don't deny that your sister comes the mogul over us, now and then. 
Wǒ bìng bù fǒurèn, nǐ jiějiě lǎo shì xiàng gè bàojūn shì de qí zài wǒmen tóu shàng.

It seems an unusual usage of 骑 (qí; to ride).  Usually, we e.g. 骑马 (qí mǎ; ride a horse) or 骑自行车 (qí zìxíngchē; ride a bicycle), but in the above example, someone &quot;rides on our heads&quot; or &quot;rides above our heads&quot;.  It sounds metaphorical.
Question: What does 骑在 someone's 头上 mean?
I found other examples:

天津：江苏骑在我们头上打。 (link)
Tiānjīn: Jiāngsū qí zài wǒmen tóu shàng dǎ.
小羊骑在你头上打一字。 (link)
Xiǎoyáng qí zài nǐ tóu shàng dǎ yī zì.
骑在你头上真好玩。 (link)
Qí zài nǐ tóu shàng zhēn hǎowán.

I also found this (我老公喜欢让我骑在他头上...) which sounds literal and sexual (among other examples).
The figurative meaning  of 骑在~头上 (ride on top of someone) is not 'to bully ~' , It is 'to dominate/ to rule over' For example: "Mr.A graduated one year before you(have been your senior schoolmate for all your college life); entered the same company one year before you and has always been your boss." -- You can say Mr.A  一直骑在你的头上 (always ride on top of you) meaning he's always been your boss or having authority over you.

If one day, you got promoted to higher position than Mr.A, then you are in fact 反过来骑在他头上 ( turn around and ride on top of him)

One more example: "他一直被妻子骑在头上" (he has been ridden on top by his wife) meaning his wife is the head on the household over him. 

As long as she always at the top of the family, no matter how well she treats him, the fact that she's riding on top of him remains.