I think you could speak up, if you do it in a non-accusatory way and don't directly refer to the weight issue. I'm sure those will really put your SIL's back up.
Rather, you might say, "I notice that Nephew doesn't seem to look very healthy and I'm worried about him. What do you think?" If she says she doesn't think anything is wrong, you might respond, "Well, I notice he's eating a lot, and I'm wondering if that's good for his health. What does his doctor think?" and then go from there.