But, from personal experience, I would not assume that she is anything like "over" losing the other child.

My own problems with seasonal / occasional depression have resembled something of a bell curve- not bad at first, then really bad, then not very bad. The threat of depression never really goes away, but the likelihood of having bad days generally dies down over time, especially if the gap from what you were missing (IE child, job, spouse, sibling, family, pets) gets filled by something else.