That's a terrible thing to do to you all. At least your FIL is sensible.
My MIL likes to show off, especially if it makes her look well off or clever. We'd decided on a wedding venue and numbers of guests (both sets of parents had wanted to pay for their own, we were paying for everything else, but hadn't wanted so many people) then it transpired that she was planning on inviting all number of random people she hadn't seen for 20+years. They were all ready inviting loads of their own friends and had 20 more than my parents. DH had never met them (it was just to show off). I pointed out she'd be going over her guest allowance and she went nuts (we were at a small family get together).
Accused me of lying, started crying and screaming. Sobbing that she wasn't allowed exactly who she'd wanted at BIL wedding so she was having her own way this time! Then tried to say I wanted her to have no one of her own at the wedding (which was BS, we'd even asked her cousin to do a reading!) I walked out. I'm not confrontational and she goes on ridiculous. She really showed herself up. SIL gave her a right telling off. DH, BIL and FIl pretended they didn't know what was going on. (But they are too soft with her)
She came and apologised afew days later, but it really soured the day. She didn't get her extra guests and we've still never met them yet.
Oh and she was loud and pompous on my hen night much to many people's enjoyment.
There was afew other things but that was the main one. My mother is so laid back and has never had a tantrum in her adult life, never mind in public.
She just has a mad fixation on making others think so well of you. People take total advantage of her and she often doesn't realise till it's too late. She is obessed with not not offending people.
Examples of offensiveness:
MIL: what type of baby hats have you bought?
ME: White M&S ones, why?
MIL: girl,or boy?
ME: unisex as I said white
MIL: but there are different styles of peak for each gender. You need both or someone might be offended when they see your baby and get their sex wrong.
ME : who the hell will be visiting us that won't have been told? If they're that offended by that they're mad and I don't want them near the baby.
MIL: 
Another example:
ME (almost 2 weeks overdue with DD1): I can't see this induction going well, so if I end up staying in hospital after having the baby please tell people I don't want visitors. (We had parent and siblings of course)
MIL:you can't say that. You'll offend people 
ME: they'll be more offended if they turn up and I tell them to piss off.
I'll be offended if they show up.
Christening for DD2
MIL: you have to invite people. They'll be offended.
ME: just don't tell them. I'll live with the guilt.
She's much better now. I just don't involve her with things.
Now that was an epic post, but therapeutic.