What makes you think that both words are from the same Indo-European source? My Latin dictionary says, "viola" is from the Greek "ion" and that's what I read in other dictionaries as well. (It looks like only btn talks about "iole".) I wouldn't know how to approach this question: same root or borrowed? And what do think about the L in "viola"? Diminutive? And what about the L in IOLANTHE? I have no idea about diminutives in Greek.