What do I say to someone who is acting this way without coming across as defending myself?
You validate. I can almost hear you screaming "BUT SHE CALLED ME A LIAR!!" However, just remember that validation is NOT agreeing. It is also not arguing/ reasoning/ disagreeing/ explaining/ negotiating. It is simply acknowledging her feelings. She calls you a liar, you say "I understand why you feel that way, that sounds frustrating. Is that how it makes you feel, frustrated?" Notice that you did not agree with her that you're a liar, in fact you aren't even discussing the content at all, you're discussing how it makes her FEEL. Do you see the difference? If you say "I am NOT a liar, and don't ever say that in front of the kids again!!!!" It will just lead to an argument. But validate, and it will calm her down and may even get her to open up about her feelings which is the real goal. You won't believe how effective it is until you try it!