OK well when I was 'halfway there' I was no longer dangerously underweight nor dangerously depressed. I was able to function in normal, day-to-day life with college/work/friends/etc. (as opposed to a few years earlier when I was in the hospital most of the time & completely non-functional outside the hospital as well). I lived on campus but got a doctor's note to avoid having the mandatory meal plan, the school cafeteria really stressed me out (not many healthy choices, food prep was out of my control, I was veg & there weren't many veg options, plus it was buffet-style which made it too easy to overdo it & eat too much... I was anorexic but I did purge occasionally & did overeat often which just kept me in that vicious cycle). So I shopped for my own foods, prepared my own meals (with just a minifridge & a microwave!), and would never eat in front of other people (sometimes I'd take a bite or two of salad & push food around on my plate to avoid arousing comments/suspicion). I hated grocery shopping but relished it at the same time. I had a ton of rules for food. Everything had to be whole-grain, vegetarian, soy-free, less than 5 grams of sugar, less than 1 gram of fat, etc... but there were certain arbitrary exceptions to those rules, and I primarily survived on Slimfast shakes and Powerbars (which make me gag just thinking about them now!!) At the time I ate a TON of dairy too (but not milk).. So a typical day would be 1/4 cup whole-grain low sugar cereal with water instead of milk, a Slimfast shake for lunch, a low-fat/low-sugar yogurt for snack, and EZ Mac with frozen broccoli for dinner. Then I might go to Friendly's with friends and eat a huge brownie sundae... Then I'd have to work out a ton & eat only Slimfast to compensate for the EZ Mac & sundae. It's hard to convey here just what that was like... I was also a cutter so if I couldn't work out (no time or too tired) and couldn't successfully purge it up, I'd cut myself as punishment. Sooooo sometime around my sophomore/junior year, when I was reading a veggie forum online, I read about how bad dairy was for you etc. and decided to cut it out & go vegan. That meant no Slimfast, no EZ Mac, & no brownie sundaes, so I was pretty much left with the good stuff -- whole grains, veggies, etc. I don't think I originally did it to be healthier (was just trying to control things more & restrict more, I guess) but what happened was I felt much more stable, my blood sugar balanced out, I didn't feel weighed down & gross after a meal, and I didn't feel the guilt of eating sundaes etc. As I felt healthier, I also started feeling really empowered because I was able to control my diet in a way that way healthy, not damaging. I liked having rules for myself, and I enjoyed creatively planning meals, and this was a much more positive way to channel those impulses. Maybe I would have 'recovered' on my own without going vegan anyway (this was also around the time where I got out of an abusive relationship and finally found a kind, caring BF who is now my husband & I think that really helped my self image & mood etc.!) but even now I feel like being vegan helps keep me from 'falling off the wagon' so to speak.
Anyway... now that I've shared half my life story & probably way more than you wanted to know... I have no idea if your sister's experience is/will be anything like mine. She could be following a different path & in a totally different place than I was. I don't think the ED experience is universal, everyone has their own reasons/methods/etc. and I had complicating issues including severe suicidal depression & later, on-going abuse... I hope this helps some though & definitely ask if you have more questions, I'm happy to provide any insight I can!!!