Younger male sibling is in love with an individual who appears to be controlling, manipulative, callous and deceitful. This individual appears to be taking on the charge of protecting the male sibling from his family under this disguise of the family would not or may not support the relationship so hard line boundaries have been established. I am interested in the psychology of why a female would feel the need to take this stance with the man's family (supportive or not).

Additional detail, he is 47 years old and has never been on his own entirely. He lived with several family members, two or three girls then eventually came back home to live with my mother then out again to live with my sister. He met this lady who is 6 years younger than him. All of a sudden, he begins to have this dialogue with everyone at a distance over social media. He has not allowed anyone to have a civil conversation with him to discuss his concerns.

In your opinion, what would cause an woman to feel this stance is necessary to take with people she does not know, never had an interaction with etc.?