It was explained recently this way. Since Islam does come from a similar route, meaning from Abraham, the Catholic Church considers that route. However, Islam is thought of as a monotheistic religion which has taken on many heresies and since the leadership has decided that common discussions are out of order it is really difficult to enter into any meaningful discussions to find a common thread connecting them with other monotheistic religions. Pope Benedict tried to address this concern during his Regensburg talks and the muslims nearly put a fatwa out on him.
For a long time I have thought that the son that Abraham had fathered, Ishmael was not legitimate because Ishmael came via Hagar and because Abraham was married to Sarah, Isaac was legit.