EastEnders cast member Kim Fox (Tameka Empson) even roped in Denise's (Diane Parish) boyfriend Kush Kazemi (Davood Kazemi) to secretly swab her in tonight's episode.
But obviously it didn't take long for Denise to guess that the two were scheming.
Eventually, Kim was force to come clean that she wanted to do a DNA test to see if they were actually blood relatives.
Upon hearing the news, Denise confessed that the results wouldn't change a single thing about their relationship.
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"Me and you, it don't matter what no test says," Denise replied, "You have been a part of my life for as long as I can remember."
She continued: "Yes I will do it, but we are sisters. It don't matter what that test says."
While the pair played happy families, confused viewers took to Twitter to slam the soap for repeating an old storyline.
One person pointed out: "I thought they'd forgotten the whole Kim/Denise not being sisters debacle. I think the viewers pretended it never happened."
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TWITTER
“Yes I will do the test, but we are sisters. It doesn't matter what that test says”
Denise
"Thought it came out ages ago that they weren't sisters," added another.
A third carried on: "Are they about to re do the sisters storyline because I remember it coming out that they weren't…"
Last year, Kim and Denise's mum Emerald arrived on the Square and told the pair they weren't related.
In heartbreaking scenes, Denise's adopted mum snapped: "She ain't even your sister."
BBC
However, Denise revealed Emerald was a twisted alcoholic and spent most of their childhood drunk.
Did Emerald lie to spite the duo?
Catch EastEnders tomorrow night at 8pm on BBC One.