In all honesty I kind of feel sorry for Mycroft. Going from being an only child to having two younger siblings in quick succession must have been very difficult. A 8/7 year age gap between siblings is quite a lot; by the time Sherlock was old enough to play Mycroft would have been a teenager. Then he ended up with a loon for a little sister and Sherlock was so traumatised from Redbeard that their relationship never fully recovered. IDK, I love Mycroft and want to give a big cuddle!
Bless poor wee Mycroft, yeah I always felt for him in fics when he was often just hopeful his little brother might eventually grow up into someone he could talk to on his own level. And then disappointed little bro turned out to be “slow”! Having a murderous little sister was probably quite challenging…
The Holmes boys childhood was always such a precious mine of different headcanons. Even once their parents appeared in the show and seemed very nice and really quite normal, it was still a fun place to explore. And now people can revisit and throw murderous small children and a non-dog Redbeard into the works; it’s a whole new playground.