SOLVED. Adult Romance: Little boy in frequently sick and admitted regularly to the hospital where his dad works. Spoiler.

[Niti]

Read this book 3-4 years ago. The main character is a widowed Doctor with kids (daughter and son). Son is constantly sick and no one knows why until the very end. I think the book is written by two authors under one pen name.

Spoiler Ahead
(view spoiler)[ Son is constantly sick because he is being poisoned by his babysitter who happens to be a cousin. The heroine is a nurse at the hospital who lost her daughter to SIDS (hide spoiler)]

[Niti]

This can be marked as Solved.
It's Abruption by Riley Mackenzie