I'm sure this was discussed earlier, but I wasn't aware of it until now when I read an earlier article. Julia was asphyxiated before she was placed in the bathtub. The article goes on to describe a medical condition Julia shared with her twin and her mother, vasovagal syncope. This condition can cause an individual to lose consciousness at moments of high distress, but was "absolutely" ruled out as a factor in her death.
In light of the other facts, i.e., no sign of forced entry, no evidence of robbery, Christmas gifts still wrapped, purse "untouched"; one could surmise that Julia's death was set up to look like an accidental drowning after she suddenly lost consciousness while taking a bath. I'm not pointing fingers at the stepdad or anyone else, but I can see now why LE might be looking for a suspect who knew Julia well enough to be aware of her medical condition, but who was unaware that an autopsy could rule out a vasovagal syncope event prior to drowning. Just a thought.
http://www.mlive.com/news/ann-arbor/index.ssf/2013/11/julia_niswender.html