Rude kids

My husband and I have a six year old, four year old and I'm currently 31 weeks pregnant. My son has always had behavior problems, he has sensory processing disorder and has been in therapy since he was 2 years old. Well a little back story to our life for context, when my daughter was almost 2 and I was pregnant with my son my sisters boyfriend passed away. My daughter does not remember him but has heard my mom and sisters talk about him so now whenever she gets in trouble or is told to do something she starts crying about missing her uncle. It's annoying because she doesn't remember anything about him and only does it when she's trying to get out of trouble but whatever helps her I guess. Now on to today, the kids were with my husband and they started bickering like normal sibling stuff and my son tells her "that's why your uncle died and you're sad". I genuinely don't even know how to address this, correct it or how to go about it in an age appropriate manner. Any advice?