The Bengali Muslims follow a custom which the Hindu's do. They bow down and take salaam from their parent's feet, their elders and when they get married to their in- laws.
So far from what I know this is not correct, and it is not a Sunnah of the prophet SAW. How do I explain this to my parents and elderly... since I have stopped doing this and they think weird of me?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Bowing down at the time of meeting or when greeting someone is not allowed even if it was done for parents or aged persons.

Imam Ahmed narrated from Anas Ibn Malik that:  "Bowing is similar to making Ruku which is not allowed except to Allah."The worst than this is touching the feet when greeting which mentioned in the question. The worst still is the fact that a woman bows down for her in-laws particularly if they are not Maharim of her.

Therefore, it is compulsory to give up this practice and act in accordance to the Sunnah of the Prophet (Sallallahu Alaihi wa Sallam). Islamic rulings should be given priority over the cultural customs and other practices. So, you have to advise them to abstain from that practice. The Prophet (Sallallahu Alaihi wa Sallam) said: "The religion is actually performing well-wishing for others (and advising them)." (Al-Bukhari)

You have to prevent them from such practice by using good manners and in a pleasant way. Inform them of the Hadith of the Prophet (Sallallahu Alaihi wa Sallam) in this matter; probably they will accept it.

Allah knows best.