What is the ruling on intense fear that relatives may fall into disbelief, and do their deeds become nullified because they utter certain verbal prohibitions?

One must cast away obsessive thoughts and busy oneself with what is beneficial. Fearing for people from Shirk (polytheism) and striving to purify their beliefs is commendable, but one should not be quick to accuse anyone of disbelief (Kufr). People should be educated and enlightened regarding matters of Aqeedah (creed) and the dangers of Shirk, as well as utterances and actions that lead to disbelief. One should frequently say "La ilaha illa Allah" (There is no god but Allah) when uttering something forbidden, and seek refuge from Shirk and ask forgiveness for it with the prophetic supplication: "O Allah, I seek refuge in You from associating partners with You knowingly, and I seek Your forgiveness for what I do unknowingly."