In fairness @Happyhappyday the thread took a nose dive when op called the double booker ‘this woman’ and went seriously down hill after sharing what her mil thinks. Throughout op has shown a high level of inflexibility, demand for perfection and called the relative ‘rude, selfish and inconsiderate’. Op is unwilling or unable to accept this to be a brain fart or clash of dates, preferring to see it as a wilful snub. It is highly predictable that her unbending attitude would hit a lot of hostility in AIBU.
Oh yes..’this woman‘...what a terrible thing to call someone. 
This is mumsnet, do you see what people often refer to people as?
And if her MIL isn’t christening woman’s biggest fan, that’s hardly OPs fault. People share info like that on here all the time. It gives context in that it explains the woman isn’t well liked.
OP is annoyed that the woman has double booked. Maybe that couldn’t be helped but to then say to OP ‘I forgot your kids party’ and only be sending her kid to the party for a bit to get them out from under her feet whilst she got everything ready, both those things are rude. Most people would know to keep those things to themselves. To have posters say that christening lady is sending her kid because she is a wonderful person trying to please OP, as if it’s some selfless act when OP has said it’s for her own benefit, well, if I was OP I’d be annoyed at those posters too, because they’re making it up. Posters also made lots of other things up too and others posters then laughed along, choosing to accept those things as fact.
We don’t know christening lady but I don’t think she comes across that well. As I said earlier, if it was a genuine case of it couldn’t be helped, the woman would have contacted OP as soon as she realised the situation, NOT said it forgot her kids birthday (even if she had.) And been a bit apologetic about the clash and the consequences for OP and OPs friend..and explained that the next available date for christening or that suited everyone was 3 years away or whatever. Because if you’re a nice person AND if you want to have good family relationships, you would know that this affects others and would want to smooth everything over.
Let’s not pretend that all these posters would be perfectly ok with spending a lot of time, effort and money for an event important to them, and then be ok with all this and ok with one of the consequences being that someone who is important to you, not being there for the whole thing.
The issues are, people decided that this was an Instagram event, probably a 1st birthday and they don’t like people who do that, hence all the piss taking. If it’s that sort of thing, it’s not my thing either, but you don’t need to be nasty to others that do these things. We’re all different.
The absolute pile on that happened and got worse throughout the thread, is extremely unpleasant. Some of the comments were nothing but bitchy. And the sheer number of posters taking the piss, quoting each other’s piss taking comments, trying to be the funniest poster, is just uncalled for. But hey, this is AIBU, the home of the bitchy people, where you can be a total cunt to others and it ok. I prefer not to be in with the cool girls here.