Back story to the argument is I have one brother who has mental health issues, very violent and has a diagnosed schizophrenic disorder.
He has been in and out of prison a lot, mainly to do with assaults etc. On the outside he sounds vile but deep down he's lost, tormented and sad. We are very, very low contact. My mum has tried to give him all the help she can but even she has had to distance herself.
Anyway, NYE, my brother went out and got beat up seriously badly (broken eye socket, cheek bone and has a bleed on the brain) by so called "friends".
My mum is understandably upset and the fight broke out outside her home. My mum is very house proud, the sweetest lady but fiercely protective of her children (me included).
DP has very extreme opinions on everything and is very marmite but also hates my brother.
Anyway, the guys who beat my brother up saw my mum outside today, sweeping leaves off her drive and having a general tidy up. One of the guys smiled, laughed and waved at my mum who is almost 70. I am furious with rage, fair enough take it out on my brother but my ^mum?
^
I told DP and he burst into laughter and said respect to them, they have balls and it's your mums fault because your brothers a cunt! 
Next thing a huge argument has blown up, I'm saying hang on - my brother is a total arse but he's 38. My mum doesn't deserve to be intimidated! More laughter by DP.... followed by well that's what happens when you have a son like that...
I've saw him in a totally different light
now I'm in bed really sad that he shows no empathy towards my mum, when she goes over and above for him. My mum left DP a little birthday cake and card on our doorstep today as she didn't want him to be without a card on his birthday (tomorrow).