I think if they'd been Grimms something would've been said by the stepfather or the uncle - considering the instinctual horror and revulsion wesen have. The mom at least; I don't think the son was old enough to have manifested his powers.
ETA: ^ agreed.
Agreed - especially in the hospital scene near the start when she's talking with her brother, she refers to him as a monster, and it seemed more than metaphorical.
ETA: ^ agreed.
My impression was that the mom occasionally saw the "monster," too, but was afraid to say anything for fear of having her sanity questioned. And maybe even doubted her own senses afterwards.
She couldn't believe she'd really seen what she thought she saw, and doubted that anybody would believe her anyway.
Agreed - especially in the hospital scene near the start when she's talking with her brother, she refers to him as a monster, and it seemed more than metaphorical.