Perfect. Let me give you a basic house training method....
Establish a routine where your pup is taken outside first thing in the morning, after each playtime and before bed. Keep in mind that dogs generally need to urinate or defecate (or both!) about 20 minutes after they eat, drink or have playtime, so pay attention to the time and make sure the dog gets outside during this time frame. When he potties outside, make sure to give him lots of verbal praise and some tasty treats (I love freeze dried beef liver...available at any petstore). If you happen to catch him going in the house (and you probably will in the beginning), clap your hands loudly to startle him, then pick him up and take him outside. As soon as he goes outside, again, give lots of praise and treats. Make a VERY big deal of it so he knows you're pleased.
Here's a link to the HSUS website discussing house breaking: http://www.humanesociety.org/animals/dogs/tips/housetraining_puppies.html
If she feels like this is stress, there are a couple over-the-counter products you could try to help take the edge off and make him feel more comfortable...
Melantonin is an over-the-counter drug that can be used to treat some anxiety issues in dogs. The dose you would give would be up to 3 mg given orally whenever necessary up to every 8 hours. More information can be found here: http://www.petplace.com/drug-library/melatonin-melatonex/page1.aspx , which includes risks, warning signs to watch out for and other bits of useful information.
Try a DAP collar. These are collars that are impregnated with a man-made version of the dog appeasing pheromone, which is a pheromone that nursing bitches give off to their pups to help them feel calm and secure. It's something that humans can't smell, but it has an amazing effect on dogs with anxiety and other issues. Although you can find them at your vet's office..you can also find them online at places like Amazon.com and Ebay for much cheaper. Just make sure they're DAP brand, as they seem to work better than some other versions.
Also, as someone who has been working in dog behavior for 20+ years, I can tell you that it's realy, really, really unlikely that this is due to the stress of a new home...especially when you've been in it for 2 months. This sounds like classic case of him possibly alerting her to needing to go out and then not being let out in time.
I am also happy to have a conversation with your step daughter about this situation. I understand that you're in a tough position, but I really don't feel like this is the dogs fault.