Ivy wrote:

I find it interesting, when that one person asked if there will be something about Mycrofts and Sherlock background or if we're going to see their mother, they didn't say no per se.
I'd love to see that and hopefully we won't get disappointed because I have so many pictures in my head about how their mother could be.

If his mother is alive and we get to see her the writers would have to answer a lot of addtional questions, too.
Did Sherlock tell her in advance about the fake or possible death situation? Did he see his mother in the night before the fall happend?
Why was she not a target? What is their relationship? Why is there no contact between them (that we know of)?
I think all this would be a bit difficult to do in the small amount of time that we have.
And the writers said that they decided that they won't add any additional characters. There are no parents in canon. I imagine that Mycroft and Sherlock might talk about "Mummy" 's reaction.