Levi Johnston's little sister HATES Bristol Palin ... calling her "evil" and a "sociopath" in a new Playboy interview. Oh, and she also gets naked.
19-year-old Mercede Johnston (above left) just WENT OFF about the entire Palin crew -- calling Sarah Palin a bad mom who's "never there for [her kids]" ... adding, "She doesn't make her kids go to school. She'd rather parade them around and put on a show, at book signings or conventions."
As for Bristol -- "Everything is about her, her, her. She has to have her own way. She'll be supersweet and then she turns into the most evil person I've ever known ... Honestly, she is the meanest person."
Mercede adds, "I didn't know someone could be so vindictive and evil ...[She's] a sociopath. She doesn't think anything she could do or does is wrong."
Mercede also claims Bristol and Levi PLANNED their pregnancy -- saying, "They were trying to conceive for months."
And for good measure ... Mercede explains why Sarah would have been a TERRIBLE president saying, "I think she'd have had a mental breakdown if she was elected [president] ... She can't even answer Katie Couric's questions."