baking soda and lemon juice down the drains - all of them...then flush with HOT water...
You can go to Home Depot and purchase a bacterio-stat - I don't know the name of it but if you tell them you need something for the smell in your bathroom - they should be able to help you.
I would reclean the toilets - make sure there is no black coming out from UNDER the rim....
The weep hole - in the sink - it's hard to get to - but put either bleach or baking soda and lemon juice down that as well - it gets moisture in there too and its dark - great breeding ground for bacteria!!
If that doesn't solve it - find out where the vent pipe is for your plumbing - if it's above her bathroom - then you might be able to get that cleaned.
If it is coming only from the bathroom - and the bleach works - I'm wondering if it's mold in the bathroom - the worst - as you would have to tear up the floor and surround to get it out...urgh...
GOOD LUCK!!!