He was a descendant from the younger branch of an illustrious family […] (1.1.3)
St. Aubert's in a tight spot in a time when "younger branch" meant less money. He's got all the prestige of the St. Aubert name without any of the cash or land that makes for easy livin'.
This sum then, he said to himself, would make this poor family completely happy—it is in my power to give it—to make them completely happy! (1.5.12)
Aw, Valancourt. He's not generous to just anyone—he really and truly feels that it's his duty to keep this struggling family on their feet.
"From this hour you must consider the Signor Montoni your uncle—we were married this morning." (1.13.3)
You don't get to choose your family in Udolpho-land—at least when Madame Cheron is calling the shots.
Such were the friends whom Montoni introduced to his family and his table, on the day after his arrival at Venice (3.3.3)
It's pretty clear that Montoni's not looking out for his family's safety. Does he even consider Emily and Madame Cheron his family, given that he's just interested in getting more cash flow?
She had no wish to reside with uncle, M. Quesnel, since his behavior to her late father and to herself […] (3.3.95)
Let it be known that blood is not thicker than water in Em's opinion… and that she can hold a grudge, big-time. Quesnel may be her uncle, but she's not interested in working that family connection.
"Was it, that you might repay my hospitality with the treachery of a fiend, and rob me of my niece?" (3.6.141)
Harsh words! But notice how Montoni calls Emily his "niece." Maybe he does understand the importance of family, after all.
"You are not unworthy of these estates, niece;" said she: "I would wish to keep them for your sake—you shew a virtue I did not expect." (3.9.73)
Maybe we underestimated Madame Cheron. After all, she's basically getting grief from Montoni just for Emily's sake—she wants to give her a proper inheritance.
The Count had a son and a daughter, the children of a former marriage, who, he designed, should accompany him to the south of France […] (3.10.4)
We don't get too many deets on this family, but we do get to hear about some possible tension between the Countess and Blanche. The Countess is a little bit of a Wicked Stepmother, if you know what we mean.
"Ah! My dear young lady!" said she, "I thought I should never see you again in this world, when I heard you was gone to that outlandish country." (4.11.7)
Theresa is the closest thing Em has to a real, live family when she returns home from Udolpho. That's why it's such a big deal that Valancourt took care of her.
The marriages of Lady Blanche and Emily St. Aubert were celebrated, on the same day […] (4.19.1)
When Em and Blanche get married on the same day, it's like a symbolic joining of their families. The de Villefortes have practically taken Em in, so now they're taking the same step together.