Domestic drama is par for the course in Shakespeare's comedies. In As You Like It, family treachery and betrayal drive the play's action and also remind us that relatives cannot always be counted on to be loyal or loving. (After all, we're talking about a play that features a duke who has stolen his older brother's title and another man who hires a wrestler to snap his little brother's neck. In fact, cousins Celia and Rosalind seem to enjoy the only loving familial relationship until the final act.)
Like King Lear and Henry IV Part 1 and Part 2, As You Like It is largely preoccupied with the unfairness of primogeniture. It explores the social problems that can arise when oldest sons inherit all of their fathers' titles, land, and wealth, leaving younger brothers with virtually nothing. Because As You Like It is a comedy, though, all of the family drama is resolved by the play's end and domestic order is restored...for the most part.
For a younger son like Orlando, the system of primogeniture has a negative impact – he's left with no father to look after him and no fortune of his own. Yet, the institution of marriage repairs everything for Orlando. By marrying Rosalind, he gains a fortune as well as a wife and father-in-law.
Rosalind's close-knit relationship with her cousin Celia is quickly replaced by Rosalind's romance with her future husband, Orlando, which suggests that marriage trumps all other familial bonds.