| Quote #1
They didn't think they could bear it if anyone found out about the Potters. Mrs. Potter was Mrs. Dursley's sister, but they hadn't met for several years; in fact, Mrs. Dursley pretended she didn't have a sister, because her sister and her good-for-nothing husband were as unDursleyish as it was possible to be. (1.3)
Mr. and Mrs. Dursley are so focused on the new family unit they've formed, and on severing any connection with the more disreputable (according to them) part of her old family, that they "pretend" such a connection never existed at all. The narrator even coins a word for it, calling Mrs. Dursley's sister and brother-in-law "unDursleyish," and really registering the specificity of how different the two families are.
| Quote #2
"I've come to bring Harry to his aunt and uncle. They're the only family he has left now."
Right away, the text reminds us of the distinction between family you're related to and people you identify with. Harry may be related to his aunt and uncle, but they're as unlike his friends in the magic world as it's possible to be. When McGonagall doesn't believe that Dumbledore wants to leave Harry with these "people," she's indicating that they can't really be his "family," because they're so unsympathetic.
| Quote #3
He couldn't remember his parents at all. His aunt and uncle never spoke about them, and of course he was forbidden to ask questions. There were no photographs of them in the house. (2.98)
This statement is particularly poignant once we find out about wizarding photographs, which are almost like small videos or holograms, showing their subjects animated and moving around. Harry doesn't even have stupid old Muggle pictures. His aunt and uncle try to keep him from developing any connection with his parents.