Its technically fraud. Lying to your insurance company so that your sister can ride your bike because M1 insurance for her is $$$$$.
The worst thing that can happen is she hurts somebody and your on the hook when the lawyers come knocking.
Any serious injury would result in a lifetime financial burden. Lets say she jumps a curb, crashes into someone who hits their head on concrete and needs 24/7 care. Your finished...
When offering scenarios on things like this one has to take into consideration the operating area of the rider. One would assume the risk of injuring a pedestrian to be lower in a rural location. In an urban area we know that pedestrians, particularly texting ones, are prone to walk into traffic like zombies.
The OP mentions "Around town" (Barrie?) and parking lots. I would list those as high risk areas. Around town could be Bayfield Street with shopping focused drivers cutting in and out of parking lots.
When drivers enter parking lots too many suddenly become boaters on a paved lake cutting across the lot diagonally, ignoring lanes.
If the insurer can dump you in the event of a crash they will do it. You could be not at fault but if the other party decides to fight, your insurer has no reason to spend legal fees supporting your case. The tens of thousands fall on you.
If the medical insurance is needed in the event of a single vehicle crash and isn't there because the claim is denied, where does the money come from for months of rehab?
It is hard for an older person to pass on advice to a younger person. The young rider wants it now. The older rider is "Been there, done that, got the tee shirt."