In the 2006 BBC Adaptation of Robin Hood (http://en.wikipedia.org/wiki/Robin_Hood_%282006_TV_series%29) Maid Marian became Lady Marian and was the daughter of the FORMER Sheriff before the real villain we know as the Sheriff of Nottingham came to power. As far as I know that is the only time she’s been the daughter of the Sheriff. And she’s never the daughter of the villainous Sheriff of Nottingham.
Sometimes she is the niece or ward of King Richard who is of course almost always absent off fighting the Third Crusade (which is historically accurate. Man basically never lived in England at all.)
Her mythology has changed over the years so I expect OUaT will take their own spin on it. We know that Keith is a womanizer and potentially also the Sheriff of Nottingham according to the full synopsis (http://www.onceuponatimefans.co.uk/apps/blog/show/25276684-2×19-lacey-full-abc-press-release) in which they listed him in the cast credits as “Sheriff/Keith” So I don’t think he’s her father. I do think he’s a trouble maker. If he’s trying to seduce Belle then it may also be that he tried to seduce Marian and thus Robin tried to steal magic from Rumple to protect Marian.
Read more about Marian here: http://en.wikipedia.org/wiki/Maid_Marion