http://revach.net/article.php?id=4239

Section: Parshas HaShavua   Category: Rashi
Parshas Mikeitz: Maskil L'Dovid - The Shevatim's Protocol On Questions About Shidduchim
Yehuda angrily tells Yosef, you were looking to libel us by asking probing questions.  You asked, "HaYesh Lachem Av O Ach?; Do you have a father or brother (Vayigash 44:19)?"  Rashi says that Yehuda asked why did you ask us this, do you have a daughter that we were interested in?  Or are you interested in our sister for your son?

Rashi does not throw out these questions as random examples, says the Maskil L'Dovid.  Rashi understood that Yehuda said this from the words, "HaYesh Lachem Av O Ach?"  How? 

He explains that when you take a boy for your daughter you want to know how many brothers the boy has.  If the father has great wealth, you want to know how many ways the wealth will be divided and if your daughter will be able to survive on her husband's share.  This would explain Yosef's probing question regarding any other brothers not present.

The question of HaYesh Lachem Av would only be proper if Yosef wanted his son to marry a sister of the Shevatim.  He, therefore, would need to ask if their sister had a father that Yosef would need to deal with, or if the brothers are her guardians and can make all the necessary decisions.