Meaning of the name Siddalee: I've looked over the internet and the closest I can come to a 'real' meaning for this name (other than 'from the movie' - which is obviously not a transaltion, but someones lack of ambition to actually try researching) is something along the lines of 'perfect meadow'...
The closest name to Sidda (that I was able to find) is Siddha, which is Sanskrit and of Indian origin - its traditionally a male name - pronounced sid-uh, it means perfected or accomplished; Sidda could (based on the movie, think of Sidda's career and how accomplished she was, especially being interviewed by Time mag) be a variance of the Sanskrit/Indian masculine Siddha.
Lee, which is English and of Gaelic origin - its used as both a male and female moniker - it means clearing or meadow...
But of course its always up to personal interpretation since there seems to be no actual name or meaning prior to the movie Divine Secrets of the Ya-Ya Sisterhood...