Long shirt or dress, no pants. They dont like messing on themselves (ususally). 

Buy M&Ms or skittles from the store and leave the bag where she can't reach on the counter as a reminder of what she gets when she even TRIES to pee or poop.  I had my kids trained (except my autistic son) in 5 days.  

I also had my kids in a long dress with no underwear.  And I put an old, plastic table cloth on the floor in the family room and put one of the baby potties in teh middle of it for the "Oh, I have to go but can't make it" instances the first 3-4 days.  After that, it wasn't needed.

My daughter loved Barney so I bought her Barney undies and would take trips to the store or an errand in them. I'd tell her to keep Barney dry and she did. We'd run to the bathroom as soon as we got to our destination and right when we got home

I had two boys trained by 2.5.  I would make them suffer the natural consequences of their actions.  If they peed their pants, they were the ones that had to find new clothes, take off their soiled clothing, wash themselves off in the shower, put new clothing on, and place their soiled clothing in the laundry basket.  Supervised, of course, but with accidents later on at 3 and 4, they would do it on their own.

the BM is trying to infantilize SS by saying "he's a baby."  Just beware of backsliding when he goes back to the BM's house.  Usually, if you are the NCP, everything you teach the child will be undone when they go back to the CP Mothership.

They all read off the same script.  I know the Girhippo had YSS in pullups at age almost 7 and he would watch TV and pee in his "diaper" well before bedtime.  I believe she had him in pull ups all weekend long and possibly in kindergarten as well.

I've lived this with my step. Keep doing the potty training in your household and eventually the other bio will realize that letting her child potty on the potty as she should is less work for her and she will miraculously jump on board. It is a control thing for them. They want to still be needed as infants need their mommy in their first years. 

As for tips: I potty trained mine in days. Keep her completely naked and limited access to wear she can get to the bathroom easily at all times for the first three days. Take her to the potty about every 25-30 minutes and have her sit. If she does nothing, make a big deal and give her a small prize like a sticker, when she pees give her a little treat like a starburst candy or something and then when she poops make a big deal about it and celebrate with both. Toddlers are very rewards-based so use it to your benefit. Get some underwear that she can pick out and that she loves (mini-mouse/etc) and encourage her not to pee on the little princess or mini-mouse or whatever. Constantly repeat "pee pee goes in the potty" and bring her with you to watch you potty. Modeling is the best learning tool for kids at all ages. 

I'm older but back in the day I bought my girls the thick winter stockings to put on under a dress, that way if they wet themselves they had to feel it run all the way down their legs. They don't like that and if they are really ready it should come really quickly once they understand no more safety net. (diapers).