Sybil (spelled this way) is a long time favorite of mine, so I think my opinion will be a liiittle clouded by bias but to answer your questions simply...
No, I don’t think it would be a burden for the child.
Yes, I do think
Sibyl will get mixed up with
Sybil (and vice versa).
Please rate my lists. All of them are ratable except for the Main list and the Surname list. :)
PNL updated 4/26
https://www.behindthename.com/pnl/174843/136906