The Hasidic woman who committed suicide four months after her sister also killed herself suffered years of depression following her forced marriage to a first cousin, a family insider told The Post on Monday.

Sara Mayer, 31, of Borough Park, who hanged herself Sunday, had a nervous breakdown on the eve of her arranged wedding about five years ago — and had already endured years of physical and mental abuse by relatives, according to the source.

“[Her suicide] was a family mental-health and abuse issue on top of being forced into marriage with her first cousin,” the source said, recalling how the union was annulled just months later.

“Ever since [her marriage], she has been in and out of mental hospitals,” the source explained. “She had been coerced by her mother’s side of the family’’ to marry her cousin. “She married the son of the mother’s sister.”

Some members of the Brooklyn family apparently didn’t find out about Sara’s torment until just recently.

Growing up, some relatives “kept calling her retarded, ugly, etc. We didn’t know this until later,” the source said.

The source added that Sara and her younger sister Faigy — who jumped to her death from a Manhattan rooftop bar in July — had been seeking advice from a family member for their depression, even though the relative wasn’t a licensed therapist.

“In Williamsburg, it was so bad that the rabbis got together and they put a poster up warning the community about her and the lack of her credentials,” the source said. “But despite the rabbis’ warning, people are still seeing her for family therapy.”

“To have lost two girls in less than a year shows that something is up with this family. It’s very sad.”

Sara, who was said to be still very observant of her Jewish faith, was remembered by loved ones Monday during her funeral at Shomrei Hadas Chapels in Borough Park.

Grief-stricken dad Israel Mayer described the difficulty of losing two daughters to depression as he eulogized Sara in Yiddish.

“Both my eyes are crying, one for each child,” the anguished father said to a crowd of about 100 people.

“I’m asking forgiveness from you if I didn’t do enough for you,” he added, addressing Sara. “I saw you suffering, and I tried my best.”

The dad went on to describe how he had tried to help both Sara and Faigy with their depression by hospitalizing them.

“We put her in this hospital, we put her in that hospital, until her soul left her,” he said of Sara, who had just been released last week from the South Beach Psychiatric Center in Staten Island, where she had been an inpatient for two years.

Additional reporting by Reuven Fenton