Unless you are using Continuity features, devices are not linked to each other. Any linkage occurs through your Apple account over iCloud. The system is designed to work that way. So I am not really clear as to what is the issue. In fact to use the tracking, texting and Facetime features you do not even need to use as Family Sharing.
If you want your wife to use the iPad for those things it would make sense for you to sign out and give it to her. Use the steps in this support article to remove your personal information from a device. --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
The iPad should Facetime regardless of who is signed in but this may be a separate issue that may even be cleared up once we figure out exactly what you want in terms of ipad usage and how best to achieve it.
Incidentally, Family Sharing may be handy to have in this situation, paricularly if children are involved. To learn how to set up Family Sharing, read this support article --> Set up Family Sharing - Apple Support