Well just because her sister is fine is kind of irrelevant. No two kids process stress or upset in the same way, and even my kids are completely different (without trauma or addiction here).
Have you asked her why she's having trouble or what you can do to help?
I used to work with kids like this and instead of telling them about hygiene and talking about smell, we would ask them general questions to see where they were at. Sometimes kids who have gone through a lot are a little more immature or socially unaware than other kids. Sometimes no one taught these kids the proper toilet procedure. Some are too embarrassed to ask. Others have sensory issues, sometimes their stools aren't fully formed (stress can have an effect on bowels) and it's harder to keep clean...
I would just ask what you can do to help and go from there. If it's more of a behavioral problem, counselling would help her deal overall.
I worked with a lot of kids and the one wipe and go is quite common. Including kids from typical families ... I had one like this at this age. Too busy playing to really pay attention.
Whatever you do (and it doesn't sound like you have - just want to mention it) don't ever shame her. That could really set her back further and is never a good motivator.
Good luck :)