Did you know he wasn't an only child? He had two siblings, one brother and one sister. Frederick Treves apparently never knew this, because Merrick didn't talk about his family at all, with the exception of his mother, whose beauty and grace he stressed to emphasize that his deformity was no reflection on her. Treves believed Merrick's mother was a fantasy construct to replace a cruel woman who abandoned Merrick, but she wasn't; she was a loving mother who died when Merrick was ten.
In particular, Treves seems never to have known of Merrick's sister Marian Eliza, who was still living at the time Treves became Merrick's benefactor. This may have been because Merrick's sister didn't measure up to Merrick's ideal of his mother or of his wishes for himself: Merrick's sister was crippled, as Merrick's mother had been. I say "crippled," because that's literally the exact word used by the primary source to describe their disability. The primary does not go into details, and because they never came to Treves's attention nothing is known about their disabilities: not their nature, or manifestations, or what limitations they faced, or if their disabilities bore some slight resemblance to Merrick's own.
Merrick was deeply proud of his left arm, which had been untouched by his disorder(s); perhaps his mother similarly took comfort in her other son William, who, unlike his siblings, was normal -- validation for her, as Merrick's mother was for him, that if not something she felt was beautiful she was at least kin to it.
Of course, William died young. The following paragraph just floored me.
In the days of preparation leading up to Christmas 1870, the Merrick's second son, little William Arthur, nearly five years old, fell dangerously ill with scarlet fever. Within twenty-four hours his condition was desperate, and on 21 December he died. The following day Mary attended the Register Office to notify his death, and the death certificate bears mute witness to the devastation she felt at the loss of her one perfect child. When she came to sign the document, Mary, the Sunday school teacher who had signed her name so confidently on her marriage license and on the birth certificates of her children, could manage no more than a cross, identified by the registrar as 'the mark of Mary Jane Merrick, present at the death."
Across almost a hundred and forty years, you can feel the woman's pain.