Glummy that's a horrible situation. I'm glad to hear your ex- BIL's GF is now doing well. I'm not going to make excuses for the officers because I wasn't there and don't have all the facts, but it sounds like they didnt explain themselves well at the very least. I'd be furious if any of my team asked someone what they did to provoke the perpetrator. 
In some cases victims will provoke perpetrators as a means of exercising control over when and how the violence is directed at them as it can keep them safer. However, exploring that has to be done sensitively and without victim blaming.
It's not appropriate when initially responding to a 999. Certainly, asking questions of a victim/witness in front of the perpetrator should NOT happen. I'm sad your family member was let down.
As an aside however, you can't abduct your own child (assuming the child was born after 2003, your ex-BIL was the registered father and there were no court orders in place). The law doesn't recognise it.
I'm not going to deny cases like the one you've described. It would be naive to think they don't happen, but they are now becoming more rare. I wish people would complain more. Despite what many think, our culture of accountability us such that these matters ARE addressed in the main, although you won't always see the outcome and it won't automatically result in anyone losing their job.
Coming back to words of advice, a PP suggested it may have something to do with victims withdrawing support part way through proceedings. That's partly true. Another reason is because I'm told the evidence base suggests words of advice have virtually no impact on domestic abusers and by making it a police decision rather than a victim decision some protection is offered to the victim - there's no point in a perpetrator trying to intimidate a victim into giving up support for a prosecution if that victim isn't supporting anyway. That said, I accept the all-or-nothing approach isn't perfect. Many of us would prefer a discretionary approach but this has been removed from us. Partly because in the past inappropriate words of advice were being given when the situation was such that much more robust action should have been taken.
Harry Miller - I think a poll of frontline officers would see 99% think the police handled this badly. (Not that I can speak for all.) Hate crime is a big problem. Particularly for the disabled (who never seem to get the coverage of other groups). It's a hot topic in the police largely guided by the College of Policing. Instead of applying a nuanced approach to look at what has happened, in typical police fashion we've formulated a one-size-fits-all 'all hate crime must be tackled robustly' approach, which is undeniably well intentioned but has been misinterpreted by some, especially when it comes to hate incidents. Most police officers groan inwardly whenever the words social media come into a crime report (although ongoing harassment of ex partners as this can be a real warning sign).
A PP said the police are institutionally misogynistic. I think the police reflect the society they serve and that society itself is structurally sexist. Police officers are drawn from society. Much of society (not me) believe women have now achieved equality, forgetting that equal rights in legislation is only the start. Attitudes towards maternity, breastfeeding, part-time working, division of domestic labour, portrayal of women in films/music videos/porn also play a huge part. Police do not count women as inherently vulnerable by virtue of their sex. Misogyny is not a hate crime. All DV victims (male and female) are classed as vulnerable but the blanket label fails to really address the issues in my view. The factors at play for male victims are different for female victims. Even though the impact on the victim may be the same, unless we understand those differences we are ineffectual tackling it in my view.
This is why I'd like to see a change to police training on DV, recognising all this. But I'm not a police chief and don't have the power. Also, the country has repeatedly shown that increasing taxes is a vote loser, even to pay for more refuges (for both sexes), harsher sentences, reform of family law to stop abusers using children to continue abuse of their former partners, or a 'no escape' approach to child support. What your left with is people like me trying to make a difference one case at a time.
Due to the £19,000 starting salary, more and more police officers are joining at a young age