The sister of a young mum killed in a house fire with her baby boy has revealed police are investigating whether the blaze was deliberately lit.

Katie Bruce, 20 and her 4-month-old son Ash Millar died in their home on the Napier-Taupo Highway when it went up in flames on March 17.

Bruce's sister Susan - who was heavily pregnant - and her partner Anthony managed to get out of the house.

Ash's father Scott Millar was pulled out of the flames but died later in hospital.

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Police have been investigating the fire.

A source told Fairfax that police believe the fire was deliberately lit, and said Millar was the main suspect.

Susan Bruce also believed her nephew's father was behind the fatal blaze.

Katie Bruce and her son Ash died in a house fire in March. Photograph/supplied
Katie Bruce and her son Ash died in a house fire in March. Photograph/supplied

She told Fairfax that when Millar was pulled out of the house she asked him where her sister and the baby were.

He reportedly said "leave me, leave me, my son, my son".

She said police had told her petrol from a lawnmower had been found at the scene and could have been used as an accelerant.

Bruce also claimed that her sister and Millar had an on-again-off-again relationship and had been involved for about five years.

Katie Bruce had left Millar two weeks before the fire and moved in with her older sister.

Bruce claimed the relationship was "abusive" and her sister often called her for help.

She told Fairfax that her sister was "smiling again" after leaving Millar, but he "always said if he couldn't have her, then no one would and he'd be the first to take her out".

Before he died Millar allegedly told Bruce's partner he started the fire.

The officer in charge of the investigation into the fire said police could not comment as the matter had been referred to the Coroner.

"Once we have completed our enquiries, the Coroner will set an inquest date".

The Herald has contacted Bruce for comment.