Well I've read all the comments and watched every video from every angle about a thousand times. My two cents:
Sofia notices the photographer (one of only a few allowed inside so likely the official photographer or one at least on good enough terms to be permitted so close)
He indicates through a look or gesture he wants to take a photo, maybe even just raising his camera ready to take a shot. Sofia, quite instinctively I imagine, pulls her granddaughters in close to her for the picture.
The girls and Sofia are all smiling and seem quite happy
Then Letizia steps in front of the photographer so the shot can not be taken. This does seem IMO quite accidental as first, its very busy at the back of the church so its understandable people are getting in the way of each other.
Sofia moves position with the girls to try and get a clear shot for the photo, she seems quite smily and happy, almost amused by it.
Felipe sees what is going on and tries to guide Letizia out of the way
HERE is where is goes wrong IMO. Letizia could of just moved out the way and allowed the photo.
Instead she stops the photo by staying in the way and trying to brush it off by playing with Leonor's hair as a way to look like she has a purpose to be standing right in front of the group to prevent a photo. Here, IMO, Letizia is in the wrong.
Sofia is having none of it and pulls the girls in closer while getting cross at Letizia as it has become clearer she doesn't want the picture to be taken. Here Sofia is in the wrong and could have just walked away but emotion gets the better of her.
Leonor pulls away, maybe just sick of the whole incident and possibly fed up of waiting for a photo that obviously now isn't going to happen.
Sofia seems really upset when she walks out the church and tried to pose again with the girls and Letizia is still there so she smiles and walks away.
They seem to be trying to look civilised before putting distance between themselves.
I appreciate everyone saying a mother is always right when it comes to her daughters but this isn't a stranger we are talking about, its their grandmother.
There is no good reason IMO not to allow the picture. They are about to walk out in public and could be photographed in all manner of combinations (Felipe, the girls, Sofia / Letizia, Juan Carlos, Felipe and on and on) by anyone who happens to be close enough. Why not just let this photographer who must be at least semi-official take a picture of the girls and their grandmother. If Letizia didn't want it made public after she could have, I am sure, prevented it from being released. Sofia obviously doesn't understand why the photo can't just be taken and I have to say neither do I. I get I am not privy to all the details of this royal family but out of anyone in the SRF Sofia seems to be most respectable and close to Felipe and his family. Yes, on reflection Sofia should have just let the whole thing drop and walked away but I think she truly dotes on her grandchildren so just wanted a photo and reacted with emotion, from the heart not her head.
I just don't see how a photo of her girls with Sofia could upset Letizia and that speaks IMO of a deeper issue. As for MC's comments it only adds to the feeling there is already tension there but really she should keep her nose out of it, very unusual for a royal, or any one with manners, to comment on twitter about a family matter.