The conjoined brothers are fraternal twins. Although the exact explanation for conjoined twins is not known, all conjoined twins have to be monozygotes, otherwise known as identical twins.
The title is from the common expression: I am not my brother's keeper.
At the end, Frank is reading "A separate peace" by John Knowles. The book is about the coming of age of two very close friends.
Interestingly, the two lead actors who are playing Siamese twins have similar-sounding last names in real-life. Timothy STACK played Frank and Jonathan STARK played Eddie.