I'll give you the benefit of the doubt about being serious. Others are quick to judge whether something on here is real or not.
If you really like her and are willing to deal with the controversy that will come with asking her out, then I would say wait until after the divorce. Technically she is still married to your uncle. Although your uncle cheated on her, IMO this would make it cheating on him. Again, they are still married until the divorce becomes final. Two wrongs don't make a right.
Plus if you ask her now, she may say yes just to get back at him. What better way to get back at him then to date his nephew. It wouldn't be fair to you, or your uncle. You wouldn't know if she was saying yes because she liked you; or if she said yes to get back at your uncle.