The ideal would be for your DH to stop saying "they're just plants," and go to his sis and say, "I don't think you realize that it's not *just* that you messed with our stuff without permission, etc. It's also that the thing you destroyed and treated like garbage is irreplaceable and had a huge emotional meaning. It's the plants that her mom and aunt planted for her during the kidney thing."
Ah, but he has to come to understand this himself before he can impress it upon SIL. And it sounds like he doesn't get it at all.
Yeah, this is the aspect of all of this that is bothering me the most. SIL did a terrible thing and apparently doesn't really get how awful her behavior was and is not apologizing and and and. HOWEVER, she is not married to the OP. DH is.
And the fact that he is not empathizing with the OP, and in fact, belittling her reaction by citing the "it's just plants" argument is (in my opinion) fueling the whole thing. I think that maybe if he'd taken cattlekid's side in the first place, sharing her indignation without exclusion, it wouldn't be hurting as much. His attitude can go a long way toward either mittigating the pain or aggravating it.
And yes, as Petticoats says, it would be easier and maybe even intuitive for DH to argue cattlekid's case to his family if he actually understood and shared her feelings.