Both very nice names, but they are incredibly similar. If you like that twin-like feeling (Molly and Millie, Jane and James, etc) then go for it. I know someone with two brothers named Dennis and a sister named Denise.
Personally, I'd look for something more distinct. Any other names you've considered?
I totally understand what your friend is saying Isla & Eli feels like a tongue-twister, the names just don't work well together. Have you got a potential middle name for Isla? If so what about swapping the names around so instead of Isla Jane maybe Jane Isla? For example.
Isla and Eli are incredibly similar. Both are two syllables with an "L" and a long "I" sound. It's very matchy, but if you only plan to have two kids I guess it could work...