When you ask Yarn or npm to install a package like @types/express, they search their own registries for the files to provide to you. However, packages from GitHub repositories or any other source not on their registries cannot be installed using the (yarn/npm) install @types/... command.
Definitely Typed is a curated collection of packages contributed by users, which are then distributed to platforms like npm. If you wish to install a customized version of sanctuary, you may need to adjust your fork's structure or create a new one to resemble a standard package format.
For reference, let's consider the twitter.js repository.
If you want to install a TypeScript package from a Git repository via npm, use this command: npm install git+{url}.git
(Refer to this answer)
npm install --save git+https://github.com/twitterjs/twitter.js.git
After installation, import it into your TypeScript files as follows:
import twitter from "twitter.js"