Relationships can be tricky enough as it is – but how do you balance things when your sister starts dating your ex-boyfriend?

This week on ‘So You Think You’re an Adult’, one woman asked how she should break it to her sister that her partner is likely lying about the number of people he's dated.

“My younger sister is currently in a relationship with a man I used to date about five years ago,” she told Moncrieff.

“Our situation was short-lived, but my sister is now long-term with him; he’s a good man and I generally support the relationship.

“However, the problem is this: my sister and I are very close and always want to talk about the ins and outs of our relationships.

“She's even asked me for sex advice about what he likes and doesn’t like.

“I find it uncomfortable, but at the same time, she’s my sister and we’re best friends.

“Last week, she told me that he had confided to her that he has had just eight lovers in his life, but he told me he’d had over 20.

“Now I know he’s lying to one of us, but I don’t want to break anyone’s trust while at the same time, I’ve an urge to protect my sister if he is lying.

“How do I handle a sensitive situation like this without alienating her?”

Tired upset woman worried about tiredness from domestic work. Image: Aleksandr Davydov / Alamy. 17 July 2019

TV personality Declan Buckley said that these two sisters need to establish some boundaries around the way they talk about this relationship.

“Look, the bottom line I have here for her is, who does she want to protect?” he said.

“If she wants to protect her sister, she has to assume that her sister is in full control of her life and that it’s actually working out with this guy.

“Maybe this guy is actually telling [the sister] more truthful things than he’s told [the letter writer] in the past; when you’re younger, you might just say nonsense.

“At the end of the day, I think the most mature kind of perspective to take would be for her to just assume that he lied to her back in the day and that it’s not really that important – because it isn’t really that important.”

Declan also said it’s “a bit weird” to be asking your sister for sex advice.

'Back off'

Actress Mary McEvoy agreed, and said this woman needs to “back off on the whole thing”.

“I think there has to be boundaries around what you talk to your sister about; this isn’t your affair,” she said.

“He was with you for a short time, he was with her for a longer time, so maybe he’s much more invested, and that’s nothing to do with you or not. [...]

“You can mature a lot in five years, you can go ‘Oh, well, I slept with about 20 people’ and actually he said, ‘No actually it was about eight’.”

Declan said the letter writer “has to not act so kind of superior in the situation”.

Main image: Man cheating his wife or girlfriend. Image: Volodymyr Tverdokhlib / Alamy. 25 May 2018