"IL told her that the reason she wont be attending is because she feels unwelcome as dil (me) has made her son (DH) cut her (MIL) out of his life shock
Good job DCIL knows otherwise.
SO now mil is playing the martyr, DH is even more shocked that his mum would stoop so low".
I feel for your DH in all this as well; he has laernt some home truths this past couple of days and the knowledge is uncomfortable to say the least. You are both victims of this woman's mad behaviour. Toxic people do not play by the "rules" which govern family interpersonal relationships.
The MIL won't go to the christening because after all this is all about her. She has "me, me, me" syndrome; very toxic and narcissistic to boot. I can imagine her own H is very much of the same bent. He is probably also narcissistic; they're certainly to be likely both as bad as one another. She won't like the fact that this time you took her power and control away.
I see she's also now played the martyr card, this is often used by toxic parents as a weapon of choice - think such people have a script. Note that you and your DH have not cut her off from your lives and the christening today; she has made the choice to distance herself. I think she's done you all a favour actually if she sticks to this. She'll probably tell her son next that he's not getting anything from her estate. Another common tactic.
Do not say anything like, "well ask MIL" - as you do not want to give woman any more ammo.
Just stick to the facts of the matter if anyone asks; the truth will out.
Do both read the book "Toxic Parents" - it does explain a lot.
Enjoy the christening today.
Attila