[Fezzik]

incestuous ĭn-sĕs′choo͞-əs adjective …

3. Improperly intimate or interconnected.

[FerociousTimes]

Thanks but I wouldn't use it though as it's an open invitation for misunderstanding and faux pas

[Fezzik]

For what it’s worth - I have never seen ‘incestuous relationships’ used in any way other than the common usage of referring to ‘nefarious bedfellows’

Edit: grammar, for clarity.