I can tell you now, you have done nothing wrong. and i can see you care about them dearly i i can honestly say they are very lucky to have you!
Sometimes, SH doesnt have to come from something huge in life, such as abuse ect, but can just be that she doesnt know how to cope with emotions. When she is sad, maybe she just doesnt know how to deal with that feeling. Guess she has learnt that SH releases endorphin's which will in turn make her feel a bit better.
As for you son, hopefully, as its only been since xmas, hopefully the SH shouldnt be as hard to stop as such, but again, learning to deal with the emotions that bring you to SH, will take a little longer. Is it also possible that maybe he is only really doing it cos he is feeling a little left out? If his confidence is low anyway, and he see's his sister getting a few more hugs, attention, or something, maybe this is the only reason he is SHing, with the right balance, maybe he will stop.
Sorry isnt very good advice.
But seriously, none of this is your fault! You children are both young, im sure with proff help and your support they can both get through this.
I wish i could have had a mum like you when i was younger, your doing great!