If you know how to play any instrument,do you already know how to play a piano?

Well seeing as you already are familiar with notes on a staff(flute, oboe, and i guess voice if you take lessons) you would at least be able to read the notes but you need to know which notes are associated with which keys. I play the clarinet, but that doesn't mean that if I tried to play my brother's sax I would get the same notes. If I tried to play a low B as if on clarinet, on sax it would come out as an F#. I also play piano myself and know that even my multi-instrumental family(at least the ones that cannot play piano) need to be taught piano from time to time because they don't know which keys are which notes. So all in all my answer is No, you do not know how to play the piano based on prior experience with other instruments.