Hi and hugs
I'm a mom of 2 DD's, one I brought into this world ( our oldest), and one my DH brought to our marriage ( the youngest).. But they are both ours. They are both adults now, married and with kids.
Have you shared your thought with your spouse? I would make this approach first and this way you are not getting into it with your step daughter mom. Your spouse can take her to the doctor as the biological parent it's within their rights. Plus this way you would know with some blood work whats going on, and take it in steps. But don't make it a big deal, something like a yearly check up or something like its for our insurance...
Stress and eating go hand in hand... some people over eat... other's can't eat. With kids and divorce she might be keeping her feeling bottled up and at that age she is not equipped for dealing with adult issue's, plus puberty is around the corner or starting. Plus with added peer pressure, or moving up to middle school or changing schools could play a factor. Plus if there is alot of chaos when she is at home with the other parent.... Creating a calm and relaxed environment when they are with you is important, that safe feeling she might open up to you more than either mom or dad...maybe taking her shopping or for a girls lunch might be in order, showing her a little special attention might be something that she really needs. Talk about everything but what you want to know... go at it sideways at this age they are like cats, you can't go straight at them, you have to circle around and let them come to you... Be consistent, and honest... she might not open up right away... but hopefully she will.
This is the approach that I had with our youngest... She would tell me things that she would never tell her parents, because I would not freak out... well in front of her anyways.
When she was 12 almost 13, we were having a family get together, swimming, BBQ'ing, hanging out. I went in the house to grab something and she came in with me, and just blurted out... " I tried smoking" I was like oh... really..... and how did that work out... trying to remain casual sounding... she was like it made me sick, and it tasted bad and I started coughing and almost threw up.....My response was "really" ... "so gross"... Then she went on to tell, who she tried it with..... so and so from school, they took cigarettes from their parents, and we tried them walking home from the bus stop... Then I was like so are you still trying smoking she was like "NO way" and she helped me carry out some snack trays. Funny thing the next time she came over, she said that her grandma had started picking her up at the bus stop
... and it was way better than walking home...
Sending pixie dust your way...