Assia Wevill, the mistress who later committed suicide, was deeply unhappy with, in her words, being treated like a housekeeper by Ted Hughes, who had promised to marry her. Elizabeth Sigmund, a close friend of Plath's, wrote of the difference in the way Hughes treated his daughter Shura (by Assia Wevill) compared to the way he related to his children with Plath, and said that the little girl was sad and quiet, seemingly well aware that Hughes didn't give her the same parental love and attention that he gave to his son and daughter with Plath.

Assia would later leave Hughes and return to London with Shura. Hughes had continued to make vague promises to Assia about setting up house with her and their daughter, while simultaneously making plans to marry Carol Orchard, with whom he'd been having affair. (He was also having an affair with Brenda Hedden, a married acquaintance.) Assia committed suicide on March 23rd, 1969, by dissolving sleeping pills in water and giving some to Shura, drinking the rest herself and chasing it with whisky. She had dragged a mattress into the kitchen of her flat and the pair were found there by the family au pair, Else Ludwig.

Ted Hughes seems to have been a lodestone of tragedy to both Platt and Wevill. Brilliant he may have been, he was also a serial philanderer, was accused by Plath (in a letter to her therapist) of having beaten her two days before the miscarriage of her second pregnancy, and was observed to have been a distant and unloving father to his daughter with Assia Wevill. Passion might make for good poetry, but it doesn't look as if it made for stable, healthy relationships.

If anyone is interested, here's an article in the Guardian that offers Elizabeth Sigmund's recollections about Plath, Hughes, and Wevill.