I just need to vent..

Lisabet • Getting married to my crazy best friend ever on June 11th 2016! 6.11.16 :) I`m so excited!
So when ever my fiancé and I have an argument and I leave or go take a bath to cool off he ALWAYS goes into the bedroom. 
Now, the problem is he has kids (which I consider my own also). His ex wife would lock herself in the bedroom and make their, at the time, 9 year old daughter watch 1-2 year old twins with autism. 
This is what pisses me, he's so willing to just leave her to watch the boys like his ex wife did! 
I try to NEVER have her watch the boys unless we're going into town to go shopping or were having a rare date night.
It makes me so mad that he's just okay leaving her to watch the twins by herself. 
She is now 11, and they are 5, still autistic.
Ug. Okay. Rant over.