Fergie spoke out as she retraced Louise's life in Germany.
"She was discarded by her husband – Ernst I, the Duke of Saxe-Coburg-Saalfeld – and sent to St Wendel," Sarah explained to the publication.
"Whatever she had done to upset Ernst – who it seems was tired and bored of her and wanted to divorce – to have to leave her two boys and never see them again was, for me, beyond words.
"I wanted to know what Louise had done so terribly wrong that she should be taken from her children on that dank, rainy 26 August day – Albert's fifth birthday – put in a carriage, discarded and written out of history."
Louise's story was said to have touched Sarah, whose own mother, Susan Barrantes, left home when Sarah was a young girl.
Speaking of her relationship with her daughters Princess Beatrice, 30, and Princess Eugenie, 29, Sarah admitted she couldn't understand how she would cope without them.
"I just don't know what it would be like for me to not be with my girls. I really couldn't fathom it.
"One of the best things I've done with my life is that my daughters and I are like a tripod. The girls are very supportive of me and I am very supportive of what they do.
"The key is to always be there, but never to wrap them in cotton wool. We work in unity and [ex-husband Prince] Andrew and I are focused on being good parents together. We are bigger than friends. We learn from each other, support each other and understand it's about communication, compromise and compassion," she told the publication.