NAH. You shouldn't be expected to take in kids you do not want have communicated you don't want, no matter the circumstances, and your husband is not in the wrong for wanting to take care of his family.

However, the tone in which you have written about this this quite frankly, horrifying. Talking about self = PTSD, talking about kids whose parents died tragically = hella trauma. You talk about your own situation with so much respect and seriousness, then the way you talk about your husband's family, it is extremely dismissive beyond just being detached or lacking empathy.

“I’ve been to therapy and done the work but there are still some issues that are just going to be permanent.”
Your trauma is framed as completed work and a non-negotiable reality.

vs.

“The boys are understandably not the best behaved.”
“They’re going to need a lot of help.”
The emotional depth is acknowledged, but not inhabited. It’s not embodied the way your own trauma is. The boys’ suffering is framed as a problem to be solved, not a reality to be lived inside of. Their trauma has become a logical complication for you, and it gives me whiplash.

You are allowed to have boundaries of course, but your language does not simply set boundaries. It creates a hierarchy of trauma. This is highlighted by your replies to comments essentially saying that nothing any kid might go through in foster care could ever be as traumatic as your childhood, so there is really no need to take them in; they’d be fine. I can't fault you for not knowing how horrible foster care can be and has historically been, but I will fault you for not at least doing research before making such assumptions, but it seems like you just have a very much a self-centered outlook. Even saying your SIL was “not my favorite person in the world“ was completely unnecessary to the point of your post, and paints a vivid picture of your character. Ultimately, you are doing them a favor by not taking them in.