As with all games on the App Store, the Family Guy game is theoretically a "threat" to TS:TO. It's hard to assess how significant of a threat, however, when we know so little about the game. It is certain that the Family Guy game will launch with less content and less features than TS:TO, given that the nature of F2P is that more content is added over time and TO has been out for almost two years now.
As many TO "die-hards" frequent this forum, there are a number of potential issues with the Family Guy game that could drive people away and keep them on The Simpsons: (1) will premium content be reasonably priced?; (2) will the servers--especially during launch--be reliable?; (3) how much of the game's content will actually be locked behind a pay wall?; (4) will there be any sort of friending or visiting mechanism?; (5) will the quests be interesting / funny?
Bottom line is, very few (if any) dedicated TO vets are going to abandon the game completely merely because something shiny and new (and most likely, crappier, at least for the first few months) has come along. New players will have to decide, based mostly on which show they prefer (there are more Simpsons fans than Family Guy), as well as user reviews (Tapped Out is consistently well-reviewed in the app store, versions to version), which game they want. We'll have to see how "good" the Family Guy game is before we go declaring it a "big threat" to Tapped Out.