Have you tried talking to her about it?
Let her polity know how you feel, being sure to emphasize that you really want to get along with her and want to get to know her genuinely. Make sure to keep it very polite, especially since you stated you don't consider her to have an malicious intent when she lies.
Maybe you guys can work it out and legitimately be able to get along!
If that doesn't work, Anne's suggestion to treat her like family, like a sibling, we don't always like our siblings, but we make it work because we have to >_<
Also, just curious, how does your partner feel about her truth-stretching habits? o=