Do you think the FP could have made up an episode where a random women knocks on the banks door and claims she is the sister of Will Smith from will's father that walked out on him, it could have made a good emotional episode in the series ?

It is cliche but it could have worked as that era of TV had numerous cliches on tv.

A long lost relative story line was already done when Geoffrey's son that he never knew about showed up on a whim. So since that was already done, I don't think it would have worked for Will to also have a long lost relative.

That is correct. I forgot about that. See? Cliches.

It is certainly possible. But this would be Will's half-sister then, I assume. I'm guessing this is the child that his father had with the woman he moved on to after leaving Will and his mom.
Seeing as how Geoffrey had already gone through that with his son, it was probably just as well that they didn't do a similar storyline again. And the thing with these long-lost relatives is that they generally only show up once and never appear again or get mentioned again.
A possible exception to this might be Cousin Oliver on "The Brady Bunch" but I think there's a difference between a relative that characters never knew they had versus a relative that they were well aware of but had just never interacted with before on the show and never mentioned so that it was news to the audience, but not the characters.
I'm not sure offhand if Cousin Oliver falls into the former or the latter category, but I would imagine it would be the latter. Of course, probably the best example of the former would a show whose whole premise was built on this- "Perfect Strangers".:) :cool:

It could have made a much better episode if a random young woman in her 20's knocks on the banks door and claims that she is the father of philip banks, that would mean that philip would have slept with another women before he met his wife vivian but later in the episode she finds out that philip banks is not her real father.