I doubt we’ll ever see or hear from a mummy Holmes. She has never played a part in the original stories, and Mycroft’s mention of her in ASiP seemed only to serve as a source of scathing brotherly banter. Her absence during Christmas in ASiB suggests to me that the Holmes parents are either deceased or estranged from both their children. After all, Mycroft did mention that - at some point in their lives - they had spent Christmases together, the fact that they no longer do so does not bode well for any future Holmes family reunions.
Bearing that in mind, I’ve really never taken any interest in the extremely hypothetical speculation of who would best portray her. There is no established character, there is not even a physical description that I know of, so one could make a case for effectively any actress.