The thing is I think almost every parent would say "I can hand on heart say I do not favour one child."
And those who don't would have an excuse why they apparently favoured that one: "they're easier" "they do more interesting things" "they need me more".
Perhaps if you look back at the emails and see what proportion mention dd1 to dd2?
I find with mine it goes round in swings and roundabouts. So one will have something big coming up and get more mentions about that, another time one will have some health issues and that will be mentioned a lot, and maybe one will have a big achievement and I'll wax lyrical about that.
I think I mention dd1 slightly less, because she's 14yo and wouldn't want some of the things mentioned that I would for the younger two. But she does get mentioned for certain things simply because she is the first in the family to get there, so for the other two it's slightly less of an issue.
Again photos on fb tend to show fewer dd1, because she's on fb, and I'm friends with parents of some of her friends -and she tends to say "Oh mum take it off, I hate that photo!". 
My dp didn't favour us generally, they were careful to be fair. However dm used to talk about db (youngest) in a totally different way. Conversations would go along the lines of "Yes, DeeWe plays the cello, she's doing very well. Db plays it too, he's got the most wonderful technique, his control of the bow is brilliant, and he's really getting the hang of the positioning."
Now in rl Db had just failed grade 1, I had just got grade 5. I think dp thought db lacked confidence, so used to talk like that to build him up.
I wish someone had said to dm something though, she'd have denied it, but I think it would have made her think, and be more careful.
Result was I grew up thinking privately db was much better than me-pretty much genius level. I said this to dm one time as an adult, and her jaw dropped and she said "well we knew he wasn't as good as you".
But talking about it to dsis and a family friend, they had both privately got the same impression. And, in all honesty, I think db did, which did cause him problems when he found he wasn't as good as he expected.
So I think it did cause problems all round.
The thing is at present ds1's achievement are probably more shareable. (ds2's biggest poo in the nappy isn't really a good one!) Also what he does is new. The difficulty comes when ds1 is 5yo and ds2 is 3yo, and ds1's achievements are still the new and exciting ones and ds2's still seem more mundane somply because he's younger.
Things are a much bigger issue when you're the first to do that, as a second child I did notice that it was much more expected and not as big a thing when I achieved something dsis had achieved.