Is a business lunch considered a bribe if the intermediary helped in securing the job?

If your relative invites you to lunch simply due to kinship, there is no harm in that. However, if their request is in exchange for an employment intercession, scholars have differed on the permissibility of accepting a gift in return for a permissible intercession. Some forbid it, while others permit it.

It is stated in "Subul al-Salam" that Abu Umamah (may Allah be pleased with him) reported that the Prophet (peace be upon him) said: "Whoever intercedes for his brother with an intercession, and he is given a gift for it, and he accepts it, then he has approached a great door from the doors of usury (riba)."

This indicates the prohibition of accepting a gift in return for intercession, whether the intercession is for something obligatory or something forbidden. However, if the intercession is for a permissible matter, it might be permissible to accept a gift as a reward, or it might be forbidden because intercession is a simple matter for which no reward should be taken.