Aaron tells TMZ ... he was FaceTiming with his friend Monday and was told his fiancee Melanie Martin regularly communicates with his twin sister, Angel Conrad, a revelation that felt like betrayal.

Remember, Aaron is estranged from his family -- in 2019 Angel got a restraining order against him after claiming he threatened to send hitmen to her door -- and he tells us Melanie knew he did not want her talking to his fam.

Aaron and Melanie have an on-again-off-again relationship, and every time they broke up he claims his family would reach out to her and see if she was OK ... but he thinks they weren't being nice, there were ulterior motives.

The way Aaron sees it ... his family is trying to gain an ally in Melanie as part of a plan to get a conservatorship over him, using her to gather information for the case.

However, Melanie tells TMZ .. she doesn’t believe his family is trying to get a conservatorship and they never grilled her on info.

Aaron says he confronted Melanie after Monday's FaceTime and claims she denied talking to his family initially, but then admitted it was true and copped to communicating with them.

Sources close to Aaron's brother, Nick Carter, tell TMZ … Nick and his wife, Lauren, have no part in wanting to put Aaron under a conservatorship, and they haven't talked to Melanie because they have no relationship with Aaron or Melanie.

We're told people close to Nick see this as another way of Aaron trying to bring Nick and his family down.

As we reported ... Aaron and Melanie split early Tuesday morning, only one week after the birth of their son.

Aaron says Melanie wouldn't tell him what she's been talking to his family about ... and that's why he decided to end their relationship. With a newborn baby, Aaron says the timing of the split is unfortunate ... but he says it's his life and he feels like he's being taken advantage of.