It is absolutely shocking. I imagine giving birth alone unexpectedly to a child with a deformity during a pandemic induced lockdown was traumatic enough to trigger a major mental health break.
There are genetic conditions linked to cleft palate and small size and other deformities. Feeding the child through a syringe was probably due to the cleft palate. What I am imagining is a mother with learning difficulties or some sort of psychosis believing she is protecting the child, not some deliberately planned cruelty.
I do find it shocking the siblings didn't find out and she didn't show any other signs of concerning behaviour. And I wonder why she didn't feel able to tell her partner.