I have a White Knight concept that I would like you address or clarify at AlphaGame.This is a good question. My feeling is that one's involvement in such situations totally depends upon the nature of the relationship. Fathers should speak out forthrightly about what they see. They should not hesitate to use their daughters' reliance upon them, particularly financially, as a counterweight, even in the knowledge that it may cause his daughter to turn against him in the short term. He should, of course, make it clear that he will be there for her when - not if - the unworthy love interest eventually shows his colors.
In regards to women in physically abusive, or controlling relationships, there will be men who will want to "rescue" them or bring them to the knowledge of their errors by speech or force. This is a continuum. On one end is the sycophantic pedestalizer (we will generously call him Suitor) who may or may not seek justification for romance in his, uhh...noble and selfless efforts. On the other end is the concerned father who wants to protect his daughter from those who would use her. The goal of the men for the woman to be out of the situation is the same, but there are non-trivial differences between Suitor and Father.
1) Suitor comes from a position of relative weakness; Father from relative Strength
2) Suitor approaches for possible personal gain, but may view his actions as dutiful; Father from Duty and Responsibility
3) Suitor has a romantic interest; Father has none*
*2 and 3 may be the same
Possibly you could chart three axes:
1) relative strength (pedestal or parent)
2) romantic interest (present or platonic)
3) responsibility for girl (none/self-imposted or absolute)
If it is true that that the preexisting nature of the relationship between a man and another person (wife, daughter, sister, son, stranger...) has bearing on his responsibility to that person, then by charting the case on the axes, you could guess the necessity of action and tactics.
The Suitor cannot ground the woman. The Father can DHSMV, but more as a way to make a fool of the romantic interest, than to set himself up as an alternative mate. Either could attempt violence, ill-advised as it may be, but the perception would change as a function of relative strength and responsibility.
Maybe I hit something here, but certainly, a man's true duty to the safety of another is according to the nature of the relationship. Can you give insight on this situation? My sister-in-law (19, out of state) is sweet, naive and shacked up with a guy with tight game who is controlling and physically abusing her. She isn't under the parents' roof any longer. I'll probably see the happy couple at Christmas. I'd like to see them apart, but I have no binding responsibility to her, or even a great relationship with her. Are there any tactics to address this or is this something to leave lie?
A brother has no similar leverage. However, he has social power that the father does not. He should relentlessly mock and belittle the unworthy man around his sister, planting the seeds of doubt that will one day blossom once the suitor fertilizes them with his inevitably bad behavior. And he should also make it clear that he will be there for her when the time comes.
A brother-in-law, on the other hand, should stay completely out of it. To be honest, in this sort of situation, I see a brother-in-law who is probably rather attracted to his sister-in-law and is likely to see unsuitability where none exists, and to exaggerate it where it does. In any case, there is no responsibility to intervene here, and indeed, to do so would rightly raise a few eyebrows, especially with the man's wife.
I'm also very suspicious when I hear about a "sweet, naive" girl who is nevertheless "shacked up". This indicates that she is almost certainly neither as naive or sweet as she portrays herself to be to her brother-in-law, in fact, this raises the question as to precisely who is the player in her relationship with the supposedly "controlling and physically abusive" gentleman in question. It wouldn't surprise me in the slightest if she had a convincing "rape" story she could produce on demand with a catch in her voice and a tear in her eye.
MT is correct. There is a continuum of sorts. But nevertheless, there is a hard and bright line between "family business" and "not family business" that should always be respected, and that line falls somewhere in between "cousin" and "brother-in-law". In most cases, if you find yourself asking "should I polish my armor and mount my steed", the mere fact that you need to ask the question is sufficient reason to say "no". Women have free will, agency, and they are legal adults in the eyes of the law. If they insist on swimming in the deep end despite not being equipped to do so, you have a solemn duty to civilization and the rule of law to let them drown.