Actually, from the way you have descrobed your husbands relationship with his mother, i wouldn't expect him to have the balls, to do as you've asked.
Here is what I would do.
I would ring his mother and state that you have spoken to Cil and that you were all invited to the christening, and that it was your husbands devious mother that has caused all this problem.
I would tell her that, although her son may be scared of her wicked and malicious ways , you are not. And as you have seen her true behaviour and feelings towards you, that you are unable to let your DD spend time with someone that can be so vindictive, so until a time as you see fit, your daughter will not be seeing her at all.
Her son is a grown up and able to (supposedly) make his own decisions, so of course if he wants to see her then you will have no problems with it.
Then put the phone down, safe in the knowledge that although your Husband may not put you first, you have shown her you will not tolerate this behaviour and you do put your child first, something she obviously doesn't.