Somebody get these folks on Family Feud.
"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," Johnston says in an interview accompanying her upcoming spread in Playboy, hitting newsstands this Friday.
But Mercede didn't stop there.
"Honestly, she is the meanest person. I didn't know someone could be so vindictive and evil," she adds about the Dancing With the Stars finalist. "[She's] a sociopath. She doesn't think anything she could do or does is wrong."
Not unlike her Tea Party-totaling mom perhaps?
Speaking of Sarah Palin, the younger Johnston (who goes by Sadie) painted a predictably unflattering portrait of the former Alaska governor-turned-failed Republican vice presidential candidate, suggesting she would be way over her head if ever elected president and would've had a "mental breakdown."
"As governor she quit on us. What does that say about her? Is she prepared to govern the whole country? Absolutely not," Mercedes adds. "She can't even answer Katie Couric's questions."
Johnston also slammed Palin for being a bad parent.
"[Sarah's] never there [for her kids]," she notes. "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."
She also contends that Palin forced Bristol's 22-year-old brother, Track, to join the Army during the 2008 campaign so word of his reputed substance abuse wouldn't spoil her chances.
"His mom made him," Johnston tells the magazine. "He did Oxycontin and mostly cocaine. He didn't choose to go into the Army; he went there because his mom made him, to get him out of the way so when she was at the convention they wouldn't know he does drugs and would think he was a patriot."
As for Bristol's allegation in her book that she ended up getting pregnant after a night of drunken sex while the ex-couple was camping, Mercede defended her older bro, saying he and Palin had talked about having a baby for some time.
"Bristol's pregnancy wasn't…an accident. She and Levi planned it. They were trying to conceive for months," she says.
Johnston reveals the younger Palin had "numerous plastic surgeries" including "her chin twice or three times" and liposuction after Tripp was born.
After claiming that she tried to mend her relationship with Bristol (who told her flat out, 'F you"), Mercede believes the wannabe reality star only got re-engaged to Levi so they could get him back in their camp so they could later attack his credibility.
"They only wanted them back together so they could control Levi, have him say he lied so no one would believe what he said and no one would believe Levi again…Everyone who knows Levi knows everything he said was true and then some," Johnston says.
As for the Palins as a whole, Johnston believes eventually "the truth about the Palins will come out."
A rep for Bristol and the Palin family was unavailable to comment.