Actually, it's not. Leah pretty much hates every female she can't use her Alpha Female role to command submission on them. And since Mercy wasn't pack, that was mostly Mercy. (And I suppose, to an extent, the human females in the pack were just afraid of Leah that they submitted because they didn't want to risk her losing control and fursploding. Leah isn't really the focus on the books so they don't focus on her as much.)

So... it's still only Mercy that she's hateful to. Because Mercy is unique and odd and... dominance issues, not authorial bias. So... evil step-mom stereotype with window trappings of it could be anyone.

The wolf-in-your-body thing, while interesting, still doesn't account for the gender discrepancies re surviving the initial infection.