This is an odd example.
It is in the mother/sisters interests to prevent somebody with similar genes to them (OP) from cuckold-risk.
Yet they will still use their rationalisations ("i'm not cheating on you if they are high status") on OP to try to shame him into staying with a woman that just had sex with a higher status male.
What could be the cause of this? Why would these women with a small evolutionary interest in preventing cuckolding be against their relative whereas the girl's friend ratted her out?
It shows how truly inbuilt the wiring towards entitlement of sex with high status males is. The whole 'sex doesn't count if they are famous and attractive thing' only is a disadvantage to men, not women. This is because a high status female celeb is unlikely to commit with the man, as there are much higher status men after her. Whereas a male celeb only needs 20 minutes to pump a baby in a woman. Its another society-wide shit-test...