I remember that incident being the other way around. I remember Edith throwing a fit because Mary had the nerve to get a haircut after Edith found out about Gregson even though Edith hadn't seen Gregson for years and Mary had and still has no idea about the depth of the relationship. Edith had an outburst in front of everyone after Mary came in smiling. She told Mary off, told the family that they were wrong for still wanting to go through with the fun weekend they had planned, and she stormed out of the room. Mary didn't snap at Edith until after Edith had gone after her--as is usually the case with the two of them.
Regarding this: the entire family went into a self-imposed exile for 6 months after Matthew's death. That meant Edith gave up six months of seeing Gregson. Then the day after receiving the news that all hope was lost, she did not fall into raptures because her sister got a new haircut. Is this overreacting? Before you answer, consider the house party in season 4. Remember when Mary, Tom, Rose and others went dancing in the hall? Remember how the dancing ceased when Mary realized that the gramophone in use had been a wedding present to Lavinia and Matthew? Everyone looked at Mary with sympathy. Seven months later, the site of a gramophone that had been given to her dead husband's dead fiancée? The poor unfortunate widow! Twenty four hours later, hating Edith because she didn't bounce back as quickly as Mary did? That bitch!
Edith can only be happy when Mary is. When Mary is miserable, the one thing that gives her solace is knowing that Edith is even more miserable. Remember Valentine's Day in Season 4? Edith got a Valentine's, and was walking up the stairs, happy. She sees Mary on the steps. Mary, who has spent six months moving silently throughout the house, rouses from her grief to question Edith's happiness. Mary made Edith feel awkward for having a brief moment of happiness, and Edith felt for her sister, not rubbing Mary's nose in it. Edith fights for survival; Mary demeans because that is the only way she knows how to survive. Her last resort has always been: at least I'm better than Edith. Remember that when you consider the sisters.