Officers said the injured boy and man were in a stable condition.
A dog, also injured in the incident, was taken to a vet but did not survive.
Det Ch Insp Stuart Brangwin, of the major crime unit, said: “Firstly I would like to extend my deepest condolences to the families of the women who have tragically died in this shocking incident.
“We have launched a double murder investigation, which may be concerning to the wider public; however, we have made an arrest and are not looking for anyone else in connection with this incident and the parties are known to each other.
“Members of the public will see a large police presence in the area while our investigation takes place. If you have any questions or concerns, please feel free to approach our officers and they will do their best to help.”
