How to share an app with friends and family only I am currently developing an iOS app for one of my family members. The only thing is, I do not want it on the App Store.

How would I be able to share this with my family member? I know that I could use Cydia impactor or with a personal developer certificate, but this would require signing every few days if I remember correctly?
If you are registered as a paid Apple developer (either as individual or an organisation), you can use TestFlight to easily share your app with friends and family. You'll need the Apple ID of the person and can add them as a tester.

You can add upto 1000 Apple IDs as tester under TestFlight and release the app. The individual members can install the app on their personal devices.

Unlike using certificates and asking members to share their device UDIDs, this approach is seamless for both the parties. The shared app build remains valid for upto 90 days from the date of publishing. This process also makes it easy to push app updates.

The family members (testers) just need to have TestFlight app installed on their device to receive app builds and updates. This also works great for Apple TV apps.

Go through the TestFlight - Apple Developer documentation to learn more.