It is assur to pasken (rule on a halachic matter) in front of one's Rebbe (Eiruvin 63). If one's father-in-law is his Rebbe and his wife asks him (her husband) a halachic question in front of her father, can the son-in-law answer since his wife has to keep the customs of her husband?
The Rambam, in the Mishna Torah, Hilchos Talmud Torah 5:3 states that if a student is asked a direct question that requires an immediate answer to prevent a transgression, he can respond to the question. To respond to questions anywhere, the student must first establish himself as a halachic authority and then he can respond to questions either (1) after his rebbe's death, or (2) with the permission of his rebbe.
When Rabbi Gedaliah Anemer, zt'l, was still alive, he often allowed his students who had moved on to rabbinic posts (teaching or pulpit) to speak from the bimah. When practical questions of halacha were asked by the congregation, they would all defer to Rabbi Anemer who usually gave permission to his students to answer (unless the issue was really a hot topic).