Some issues with Fowler's novel - strengths and weaknesses

The issues before us in reading Karen Joy Fowlers's We Are All Completely Beside Ourselves: Is it credible that a family could raise two children and a chimp as if they are family equals, treating each the same, and that the children, particularly the young girl who is the same age as the chimp, would treat the chimp as a beloved sister? Of course that's the experiment that Rosie's father, the research psychologist, engages in or rather puts his family through - and in terms of this novel, the experiment worked, that is, the entire family treated the chimp just like a human sibling or daughter - not like a pet. We see this as the recovery of the lost "sister," Fern, becomes the dominant theme in the lives of the two "surviving" children, Rosie, the narrator of the novel, and older brother, Lowell. Within the scope of this novel, I can accept it - but I'm not sure it's realistically possible, that the chimp would be accepted as anything more than a beloved pet. We all mourn our lost pets, dogs especially, but not to the degree expressed in this novel. But of course what we don't know: why did the parents get rid of the chimp, and in such a secretive, nocturnal manner? It's hard to believe there wasn't some major, precipitant cause, and that the children, especially the older brother, would know about it: attack, destruction, something. I feel the narrator, which is to say, the novelist, is holding out on us here: surely Rosie by this time in her life - college age - knows why her parents got rid of Fern; and surely she can't be surprised to learn that Fern wasn't sent to live on a farm, a preposterous palliative that she would have seen through pretty quickly, smart girl as she is. Fourth issue: can we believe that 18-yeara-old Lowell, a seemingly well-adjusted high-school senior and basketball star for that matter, would suddenly disappear and head off to live on his own, beginning a peripatetic life of maybe 10 years or so, without any family contact other than the occasional mysterious post card? Perhaps he's not such a normal kid, I can accept that, but such behavior - on his part, and on the family's that more or less lets him go after a cursory effort to track him down through a detective agency - is beyond the ken. A strange family, no doubt - but as we have not yet (half-way through) actually "met" either parent, it's hard to know how strange they really are or may be. Rosie is a tricky and unreliable narrator, which is part of the strength of this novel, but also part of its weakness - too many holes and gaps that feel like narrative stratagems.