My boyfriend is waiting for his younger sister to get married since they will not allow male kids to marry before sisters get married. what is the reason behind this culture in Hinduism?
To my knowledge, there are no such rules mentioned in the scriptures, according to which a sister must be married off first.
But there are some other instructions from which, this scenario (of the sister getting married off before her brother) naturally follows.
For example, as per the Manu Smriti, a 30 years old man should marry a maiden aged 12 years.
Manu Smriti 9.94. A man, aged thirty years, shall marry a maiden of twelve who pleases him, or a man of twenty-four a girl eight years of age; if (the performance of) his duties would (otherwise) be impeded, (he must marry) sooner.
Now if this instruction is to be followed, then it is quite obvious that the girls in the family will have their marriages before their brothers'.
According to Manu Smriti and similar scriptures, a (twice born) man must dedicate quite a good number of years of their lives to living in the Guru's house and studying the Vedas. This is called the Brahmacharya Asrama and the Gurukula Vasa. He is allowed to marry only after passing through this stage.
An initiated person should reside as a student in his teacher’s house for forty-eight years, or for three-quarters of that time, or for half that time, or for one-quarter of that time; the minimum is twelve years. A student who seeks to acquire vedic knowledge may not reside with anyone else. (Dharmasutras of Apasthamabha 1.2.33)
Forty-eight years, according to ancient practice, is the period of studentship for studying the Veda; alternatively, twenty-four years, or twelve years per Veda, or at least one year for each book, or, given the uncertainty of life, until he has learned it.A vedic text states: ‘A man should establish his sacred fires while his hair is still black. (Dharmasutras of Baudhanaya 1.3.12)
And, the respective ages for initiations, for the three caste men are as follows:
Manu Smriti 2.36. In the eighth year after conception, one should perform the initiation (upanayana) of a Brahmana, in the eleventh after conception (that) of a Kshatriya, but in the twelfth that of a Vaisya.
So, even if we assume, that they dedicate only 12 years to the student phase of life, a Brahmin man can not marry before he is 21, a Kshatriya before he is 24 and so on.
But, as per the same texts, for a girl there are no vedic studies so no such compulsion for her to wait a certain number years before she could marry. In fact she should be married as soon as she attains puberty.
So, the manner in which these rules are set, and if they are to be followed, it is only natural that the sisters in a family will have their marriages done before their brothers. Because, the age at which a man can marry is set much higher than the corresponding age for the woman.
So, I think in ancient times, when these rules were followed, naturally sisters of the family used to get married before their brothers. But gradually, there came a time, when people forgot the real reasons behind those injunctions but still kept following the rule, thinking it is only necessary to marry off the sister earlier. This I guess what must have happened. Because as I have said, there is no such rule mentioned in the scriptures.