stay -that's exactly my point! Thank you.
OK, have seen my friend and the story is a little clearer!
The groom has a slightly younger brother only, no sisters. This is 'good' in that MIL has to be careful how much she screws with her son's wedding as she won't have many, if any more to go to. She can't afford to cut one son out, as it were. This younger brother is close to his bro and the bride.
My friend says trouble has been brewing for weeks; the MIL told the bride that it was 'disgusting' that the bride's brother (17), uncle (hippy, free-spirit) and grandfather weren't sitting on 'the top table'. There is no 'top table', just another round table. None of the listed particularly want to sit on this fabled table, bearing in mind the bride's family are close and all grown up i.e. won't be the ones spoiling the day! The groom then sheepishly asked my friend to help them reorganise the seating arrangements as Mother Is Not Happy as, despite her assertion that weddings are 'all about family' her original family and her DH's were under no circumstances to sit together..... One invited family consists of mum, dad and 3 adult DC (in their 20s). They had to sit all together, side by side, no putting the 20 year olds with other 20 year olds on another table (nothing 'wrong' with any of these siblings!); 2 twins had to sit next to each other- they're 54....Uncle Soandso had to be on the same table as but not next to someone else... At one point one round table that comfortably seats 8 had 12 on it, and the next had 5, etc etc.
Anyway, to keep the peace, my friend helped with this, and thought the whole thing had blown over. She asked the groom a while later how it had gone and he said, in embarrassment 'Mum's still not at all happy, and she's furious that we've invited my cousin and wife whom we see and socialise with all the time and not another cousin I last saw when I was 6'..
Fast forward to The Big Fight. Bride and Groom went over to his parents to 'discuss' the non-sanctioned invites. Bride said to MIL 'You have to understand that this is our day, not yours, we've invited whom we want to be there, we don't want or need 'randoms'; those who are coming have been paid for (by the bride, groom and my friend, not them at all) and the room is now full' -at which point, rather weirdly, MIL suddenly starting prancing around the room, waving her arms theatrically around, saying 'Oh, welcome to makes this day all about HER!' etc. Apparently she went on for minutes! Then the bride said, well, yes, my wedding day is all about me, actually, as opposed to about you. The father didn't join in on any of this and the younger brother was trying to calm mother down. Then MIL said 'My son could do so much better than you, you know! You've changed him, since he's been with you' (note:since he was 17! You'd hope he would grown up in 7 years!); 'YOU haven't even got an intact family half of yours won't even be there' (only the bride's father isn't invited) to which the bride said, verbatim 'If you say that again, you'll regret it' (not that she'd deck her!) at which point father sprang up, frothing at the mouth and squared up to his 5'2" future DIL, and the 2 sons had to restrain him! 
They left with MIL shrieking after them 'We're not coming to the wedding and neither is the rest of our family!'...
As soon as they got in, the younger brother texted to say how embarrassed he was and how bang out of order his parents had been.
Anyway, 2 days later she texted her son the groom and said 'After that disgusting display the other night, I hardly want to speak to you but there's something I need to see you about' to which he replied- 'The next conversation I have with you will be your apology to me and my fiancée.'
Well done, that man.
My thinking is now that yes, she and her DH may not come to the wedding but that she seems to be unlikely to be able to prevent any other family members attending; my friend thinks she'll 'limit' herself to inviting the hoards of cousins to the evening do which is still completely unacceptable but at least won't be embarrassing etc at the church and reception; and we'll see what happens about an apology!
My friend's mates (us) who've all been invited by the bride as thanks for supporting the family through a shite few years have all begged to be allowed to sit in every other seat between his parents and difficult family members. We'd have a hoot, subtly pissing them off!