I know that SIL fonts can be used for printed books, but can they also be used for digital (epubs)?
Current SIL fonts are distributed under the SIL Open Font License, and can be distributed with digital books and epubs, but the details of how to do that properly depend on the technology you are using. See OFL-FAQ questions 1.15 and 1.16. It would probably be a good idea to read all of the OFL-FAQ section 1.
If you are making epubs with Bloom, it will detect that embedding SIL fonts is permissible and do so automatically, so the books will have the fonts embedded and the pages will indicate that they should be used. Whether it actually happens, however, is up to a particular epub reader application.