My mother's sons were angry with me;
they made me keeper of the vineyards,
but my own vineyard I have not kept! (1:5-6)
Yep, the first we hear of these guys, they're angry. Not off to the best start.
Back in the day, family was hugely important to marriage—family ties, money, and yes, siblings, were a big deal. Remember in Genesis how Jacob wanted to marry Rachel, but Rachel's father gave him her sister Leah instead? Yeah, that stuff happened. And here in Song of Songs, the bride makes it very clear that her family is preventing her from marrying the man she loves.
But they do come around, mostly to serve the love story we've grown so attached to. In their final appearance, the brothers declare that the bride is too young to be married, but acquiesce to her wishes:
We have a little sister,
and she has no breasts.
What shall we do for our sister,
on the day when she is spoken for? (8:8)
What we have here is basically a conditional—and lukewarm—of the marriage. Because of the importance of family approval in marriage, this concession sort of validates the bride's love for the groom and allows it to survive the gauntlet of real-world family concerns.