"To be fair this is the first time MIL has ever done this. So while i don't believe the strange illness i will accept the apology and move on."
On reflection, it sounds like she overstretched herself so she could not make good the clippings/mess/havoc at yours before you came home.
Could it be she started this horticultural venture to be helpful/creative/interfering/daft/PA gesture (delete as required), & with boisterous tornado DN in tow, she ran out of steam?
Is long hours with DN too much for her? Even more too much whilst you were away? Might she have brought DN round for change of scene, safe garden to play in & fresh toys?
How are the boundaries between yr MiL & SiL? Would she hesitate to reconfigure yr SiL's home or garden? What is yr SiL's opinion about supervision of DH during the hedgehacking ?
Once the heat of your ire has passed, I do wonder how far you & DH will get establishing firm boundaries, particularly if they are smudgy between MiL & SiL.
My InnerMinx suggests asking for the house key back, assuming she has made a copy, & still change the locks...