Why is it that my stepmom lies?

Ok, here's the deal. First off, I am 38 (not a child). So she tells everyone and always has that I am just like her own daughter. Which is BS. She treats her daughter totally different than me. But, she treats me pretty good. Anyway, I have two small children & so does my step sister. My kids are good kids. She has recently told me that she couldn't babysit them on a particular night I asked because she is watching my step sister's kids. Well, this was for next weekend --- all the sudden she has my step sister's kids this weekend and says that it was last minute and they changed their vacation to this weekend --- which I know is not true. I hardly ever ask her to watch mine. Why did she lie? Why does she lie? I really don't understand that at all.