First, let me say I am so sorry for your loss. No parent (or sibling) should ever have to know that pain. I can't begin to imagine it and I suppose it never entirely fades.
I think you've really hit on something there, though, especially if the nail biting started shortly afterwards. I bit my nails to the quick throughout my childhood. I hadn't lost a close family member, but I was always a nervous kid ... always worried about making a mistake. For me, the nail biting was about getting those rough edges all perfectly smooth/even. I'd find one end that was too long & bite it down, then it would be too short, so I'd go after the other end until I hit skin. As soon as they'd grow out just a little bit, they'd be uneven, so I'd start all over again. I think it was really just about feeling out of control ... if I could just get those nails perfectly even, I'd feel OK. Maybe your daughter is experiencing some anxiety (I'm pretty sure I would be, if I lost a loved one so tragically) and has a similar need to make some small part of her world "perfect". Has she been seeing a grief counselor? If not, maybe that would help. Even if she doesn't seem distraught, she may just be putting on a brave face, because she doesn't want to make you feel sad again. Perhaps her pediatrician can make a referral or maybe her school has some resources that might be available.
As for the lemon juice, special nail polish (tastes bitter), etc., I doubt you'll have much success with any of that, unless you get to the root emotional source of the problem. My parents didn't do the lemon juice, but it stings, anyway, when you bite your nails that far back & that never deterred me. My parents tried the nail polish & I'd just bear the bitter taste until I managed to scrape off all the polish. It wasn't until I was in my late teens that I managed to kick the habit & it was really hard & I was really motivated (vanity will sometimes work wonders). I still backslide, sometimes.
Anyway, good luck. My heart really goes out to you and your daughter.
-S. M.