I will try to keep it brief.
Dbil has 1 Ds aged 17 months. He is fantastic with this child and loves him to bits.
His ds's mother is a - God, I can't even think of a word for her. She is utterly vile.
She abuses dbil physically and verbally. He has never raised so much as an eyebrow back.
She has a dd from a previous relationship who was taken into care but returned to her when ss realised what a great guy dbil is. He got her flat in order, got her in order as much as he could, ensured she visited her dd when she was meant to etc. he cared for her dd like she was his own.
She doesn't care two hoots about either of her children. The dd is always palmed off on some relative or another, the ds is always with dbil. And I mean always. He does everything. Gets up in the night with the kids, cooks, cleans, washes, irons, shops, sorts out bills etc. she does nothing but go out and see her mates, gets pissed and sees other men.
She spends no time with her son. When he cries, she calls dbil. She won't pick him up, won't comfort him, wont clean him up, wash him or his clothes, won't change his happy, won't take him anywhere.
He has recently left her, taking his ds with him. She has decided that she wants to see the ds - when its convenient and she has nothing better to do. So, dbil took ds round to see her and said that he was popping out for an hour and left. She ran out with the child as he was driving away, put the child - no shoes or coat - ON THE PAVEMENT - and walked back into the house.
Dbil looked in the rear view mirror, saw her going in and his ds crawling up the pavement alongside the road. How the hell could anyone do that to a child?? 
Since then, she has threatened to leave the child alone in hs buggy on the high street and walk away if dbil did not come and take him immediately as she "could not cope". Bollocks. She wanted to go and met her new bloke in a pub, and couldn't do that with her ds.
She is a complete bitch and should not have responsibility for a fly, let alone 2 children.
I really really want to say something to her - but should I just keep my nose out?