We have relatives who want to follow our Instagram (just for viewing pics) but they have neither an Android nor an iOS device. Most of the Instagram web interfaces don't have the option to register a new user (they allow existing users to login only). Is there any way they can register on the web?   
How to register a new Instagram user via web site
instagram
No, there is no way to do that. A next user (e.g. yourself) must grant access to a Android or iOS for your relatives to sign up. I would look over the terms of service here.