Nonnies, I've craved something like this for ages:
Years ago, alpha!Mycroft found omega!Sherlock while the latter was in heat and high. The drugs messed with Sherlock's scent (or something) so that Mycroft reacted to his sibling and vice versa and Holmescest happened. For whatever reason, the resulting pregnancy wasn't terminated. The brothers end up having a difficult relationship. The (healthy) kid either lives with Sherlock or goes to a boarding school in Switzerland or wherever. Mycroft made sure that nobody knows it's his child and is terrified of the Sherlock-drugs-combination because he fears Holmescest will happen again when Sherlock is high and they're alone.
Then shit hits the fan. Magnussen finds out? John finds out? Holmescest happens again.
Alpha!Mycroft and Omega!Sherlock procreated
Years ago, alpha!Mycroft found omega!Sherlock while the latter was in heat and high. The drugs messed with Sherlock's scent (or something) so that Mycroft reacted to his sibling and vice versa and Holmescest happened. For whatever reason, the resulting pregnancy wasn't terminated. The brothers end up having a difficult relationship. The (healthy) kid either lives with Sherlock or goes to a boarding school in Switzerland or wherever. Mycroft made sure that nobody knows it's his child and is terrified of the Sherlock-drugs-combination because he fears Holmescest will happen again when Sherlock is high and they're alone.
Then shit hits the fan. Magnussen finds out? John finds out? Holmescest happens again.