He said he never fought back, but snapped the day of the shootings.
However, he broke down when Ross described how the boy had shot the girl four times, twice in the head.
The 17-year-old testified on his own behalf during the continuation of a sentencing hearing in Brandon.
The youth was 14 when he shot and killed the 43-year-old woman as well as his sister at a St. Lazare, Man., farmhouse in August 2007.
The teen, who cannot be identified under provisions of the Youth Criminal Justice Act, was initially charged with two counts of first-degree murder.
A trial was set to begin Jan. 11, 2010, but it was delayed and on Jan. 14 the teen pleaded guilty to second-degree murder in a Brandon courtroom
The Crown is seeking an adult sentence for the teen, meaning he would have to spend 10 years behind bars before being eligible for release.
Abuse allegedThe teen told court in a quiet voice that his adoptive mother often hit him with objects like belts, cables from a videogame console and, on one occasion, a shovel.
It was during one of these beatings that he grabbed a rifle from his parents' room, loaded it and shot the woman, he said.
Under cross-examination from the Crown attorney, however, the teen couldn't say why he also shot his sister four times. He broke down into uncontrollable sobs when the prosecutor pressed him for answers.
The crown pointed out the boy's testimony in court differed from accounts of the crime he told counsellors after the incident.
The teen's claims of being abused are disputed by his mother's family, who testified last week the woman was loving, caring and in no way abusive.
Outside the courtroom, family members showed the media pictures of the family from happier times and said both of them were loved dearly.
On more than one occasion, visibly upset family members present on Monday for the hearing had to leave the courtroom during the teen's testimony.
The hearing continues this week.
I think the teen is telling the truth about being abused. You don't lie about serious stuff like that. I have sympathy for him and completely understand why he killed his mother, if long term abuse did occur. Instead of the Battered Woman's Syndrome defence, there should be an abuse defence which is universal across all ages and all genders. The teen probably was in a mental state of learned helplessness, where he saw no way out and saw no other reasonable options. I believe this teen should NOT be sentenced as an adult. Rehabilitation and reintegration need to be the main goals and focuses for teens. The maximum youth sentence for second degree murder is 7 years, with 4 years in custody and 3 years under conditional supervision in the community. I believe this teen should be sentenced to 5 years, with 3 years in custody and 2 years under mandatory supervision in the community.