I didn't understand if or when the boy stabbed the mother and when the uncle was killed
Yes, he did stab his mother. I get why you are confused. Tsukagami broke his stepdad's bone, so he presumed Haruki's mother would go to the hospital to take care of him, while she did not, or could not.
After putting Haruki to sleep, he did think of what the boy said:
and asked his acquaintance to send police to the house to check:
.
"A man" being reported here is Tsukagami himself. Since the police would never doubt a ten-year-old boy to do the stabbing, so yeah
Hope this helps. His uncle was killed a long time ago, that's all I can say for now.