I am able to save $300 each on 2 tix to Barcelona from the midwest by booking one roundtrip ticket from a regional airport through Detroit onto Toronto. Then later in the day book a ticket from Toronto direct to Barcelona.

My question is that i am aware of Canada's very strict laws regarding any previous charges in the states. I am traveling with my sister and she received a DUI charge 3 years ago. I know if she wanted to just visit Canada she could not get in. What happens with flying though? I'm assuming since I want to do this on 2 different tickets, when we arrive in Toronto we get our bags and re-check with the other airline going over to Barcelona. But doesn't the Canadians have to "allow" us through? Which I'm thinking my sister couldn't get through.
Thanks in advance.