"Me sniffing her period and getting those pheromones triggers my brain to start producing them," Jessica explains

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Just when Extreme Sisters audiences think they've seen it all, these sisters are taking it to the next level.

In a PEOPLE exclusive sneak peek at Sunday's episode of the TLC series, Jessica has that "before period feeling" — and in an effort to help jump start her cycle, she sniff's her sister Christina's "pang pang" area.

"When our periods are in sync, we're emotionally in sync and we're more connected," says Jessica in a confessional, as Christina adds, "We PMS at the same time."

While Jessica finds it helpful that they PMS together, Christina disagrees: "No it doesn't, 'cause we're crazy together."

"Yeah, but I can deal with you better when I'm crazy too," Jessica responds.

While folding laundry together, Jessica tells Christina, who just started her period, "I need your pang-pang" because she's "getting crabby" and "cranky."

Extreme Sisters; TLC
Christina and Jessica
| Credit: TLC

Obliging her sister, Christina scoots over on the couch and Jessica bends down close to her sister's private region. "I need the whiffy whiffs, you know, the pheromones," says Jessica, "the aromas."

For the siblings, science explains their reasoning behind the act.

"We share the same DNA, so obviously we're going to share the same pheromones. It's basic science here. Me sniffing her period and getting those pheromones then triggers my brain to start producing them," Jessica explains.

It's also reciprocated. "I'm not gonna lie, I do it to her sometimes too. But I do it when I'm like, 'Oh my god, I'm super late, like, I need to start it because it works,'" says Christina.

Extreme Sisters; TLC
Jessica
| Credit: TLC

Furthermore, Jessica wants Christina's boyfriend to know about the sisters' sniffing.

"This is the stuff that I want like, John to be around," she says. "John has to see the real us, he needs to know the real you. He needs to see it all."

Extreme Sisters; TLC
Jessica and Christina
| Credit: TLC

Seeing the "real" Christina also includes her profession as a psychic — something she hasn't revealed to John in their eight months of dating.

"So I've been with John for eight months and he still does not know that I'm a psychic, so I usually go to my sister's when I talk to my clients. I keep my tarot cards hidden under the bed with my crystals. I put them in my laptop bag and I leave the house," says Christina.

Extreme Sisters airs Sundays (10 p.m. ET) on TLC.