What's "normal" in a break-up where kids are involved?

[SingingPigs]

Realistically, do people manage to do things amicably in relation to the kids?

DSis's Ex-P is screaming abuse at her in front of their children, telling them their mother is a very bad and heartless person, using them in any way he can to "punish" DSis (e.g., arranging to see the DC then cancelling, making them very upset and angry with DSis), threatening to leave and never see the kids again (because he claims it's too hard for him), threatening DSis and badmouthing her to all and sundry... I could go on.

DSis is broken, and seems to think it's her fault (from most reasonable perspectives she is in fact the "injured party"). She also seems to think that being hurt, and the fact of the break-up, excuses his behaviour.

But people do manage to split without this happening, don't they? She would not be unreasonable to hope for better behaviour from him?

Yeah people do manage it, but if one of them doesn't want the split then it's hard - it's hard anyway.
All you can do is try your best to do what's right for your kids, regardless of what anyone else does.
Time helps.