Difficult Talmudic Passage

Let's learn a shtickel gemara [קידושין יב] and let the budding Talmidei Chachomim think about it.

"A certain man betrothed [a woman] with a black marble stone. Now, R. Chisda was sitting and appraising it: if worth a perutah, it is well;  if not, not.... Said his mother to him: But on the day he betrothed her it was worth a perutah!  It does not rest entirely with you, replied he, to render her forbidden to the other man. For is this not comparable to the case of Yehudis, R. Chiyya's wife, who had severe travail in childbirth.  Said she to him: My mother told me: ‘Your father accepted kiddushin on your behalf [from another man] when you were a child.’
 He replied to her: It does not rest entirely with your mother to forbid you to me." [Soncino trans.]

This gemara is very difficult to understand. Rav Chisda said that "it doesn't rest entirely with you to render her forbidden to another man" which implies that only because there is another man involved the mother is not believed but if there wasn't another man involved she would be believed. But we know the rule is that אין דבר שבערוה פחות משנים so she shouldn't be believed in any event. Also, why do we need to prove our point from the case of Yehudis, the rule is simple - אין דבר שבערוה פחות משנים?

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