+1, this is poised to create issues and potentially ruin a few relationships.
OP’s sister is used to Apple services and not even other payed cloud services come close to the level of integration Apple provides. It just works, is a real thing inside the Apple ecosystem and anything the OP might get will be inferior and she will complain.
On the day the service is down or something doesn’t work / some update breaks the sync or wtv she’ll just be there with an “entitled atitude” pressuring the OP to fix things.
This is like one of those situations where you have a LOT of work setting up and managing something and people will never recognize the work, help, split the bill or be patient. People are so expected tech to “just click a button” and everything just works and is free that they aren’t even able to understand the complexity of what’s behind it all and the amount of work it is required to get “a simple file sync” to work.