We don't know if she knew before the scandals broke but there is no reason to think she didn't believe them after they broke.
In terms of her standing by him, this is not really a historian question in the sense that there is not really a definitive answer if you're looking for one. However one narrative about the Clintons that is popular is that they were both very politically active and ambitious, and that his fidelity may have been less important to Hillary Clinton than their political careers and the presidency.
However it is impossibly to know the truth as we cannot get into the mind of Hiliary Clinton to know how it effected her personally or how important it was to her. Perhaps she will release a memoir or diaries will be released after she dies for future historians to try to grapple with this.
It is also a highly politicised topic as the Republicans led by Newt Gingrich tried hard to weaponise the events around Monica Lewinski and impeach Bill Clinton. This taints the perspectives on the issues on both sides of the political divide.