Ophelia, I agree with the above posts. I think you are being too nice to him (and I really don't mean that as a criticism)
To be honest Ophelia ... from what you've written ... everyone in real life is being too nice to him.
The sense of outrage on this thread is palpable. This bastard has behaved off the scale bad - despicable, almost unbelievable in some ways. In a top ten of MN all time bastards, - he's got to come pretty close to the number one spot. You can sense from every post of everyone reading how utterly disgusted they are with him.
And yet ... there doesn't seem to be that reaction from anyone you mention in real life? You say his mother just accepted everything he told her after 12 years of being married to you and rang you and said 'we will be seeing less of you in the future'. No regret, no disappointment, nothing.
Your dad did the handover and said your ex was pleasant and friendly and your brother is still 'friends' with him on facebook. You make no mention of the reaction from anyone else. If you were a friend or famiy member of mine I'd want to rip his head off and shit down his neck (knicked that from someone else on here) I'd be that fucking angry at the appalling way he has treated you.
Yet in little over a fortnight, this fucker has waltzed out of his marriage leaving three year old twins, straight into the arms of his mistress. He's seen several times in public (including by you kissing in the street apparently on their way to a restaurant) by your brother in town with her child and by various friends in the pub. His job is unaffected, his reputation appears unaffected, he's flaunting his new relationship left, right and centre ... and no-one seems to be particularly angry at him - apart from the respondents to this thread?