Golly, I hate to say this, because it's dragging in other family. However, if DH is that cowed by his family and, you, OP, don't feel that you can confront SIL yourself, how about having YOUR Mom call SIL and say, "I heard you tore up the plants I paid for and placed in my daughter's yard to commemorate her kidney transplant. I just want to let you know I don't appreciate you doing that. I don't care if you were trying to help, you still destroyed my work and caused my daughter stress that she just doesn't need."
SIL might tell your mom to go do something interesting to herself, but it also puts DH's family on notice that their antics are not going unnoticed by the world at large.