Why would the MC ask about their father? You are told that Gracie's family abandoned her when she was pregnant, and that she struggled to bring the girls up herself - it is fairly obvious that there was no father present. It would be perfectly natural that they had no one else to turn to.
The MC may not remember Gracie, but he has enough information that they were obviously close friends at school, and that she regarded him highly enough to consider him the best guardian for her daughters. As the MC is presented as a thoroughly decent bloke, I don't see why he would not be flattered by her trust and accept the girls.
Half the problem is that people click through without reading the text and then make wrong assumptions as to what is going on. This is what IT professionals refer to as a "pic-nic" (Problem In Chair Not In Computer).