In response to your question, I say:
The issue of the marriage of Caliph Omar to Um Kulthoum, daughter of Ali and Fatima ( peace be upon them), is very vague, as the news about it are conflicting, the stories received through the Sunnah say that the marriage took place, while the shiite narrations deny it, and while the first are weak, the second are considerable.
Some news also says that she is not Fatima's daughter, but ali's daughter from another wife, and some news says that she is the daughter of Abu Bakr, from Asma Bint Ommais, who was married to Ali (peace be upon him) after the death of Abu Bakr, and lived with her kids in Ali's house.
It is worth mentioning that the presence of a daughter of Zahra (peace be upon her) in the name of Um Kulthoum is not historically fixed, as a number of historians argue that Um Kulthoum is Zainab, and was called Um Kulthoum.
As it is well known, Zainab was has not been married to any one other than Abdullah bin Jaafar al-Tayyar.
On the other hand, The way they mention omar's marriage to Um Kulthoum, that Imam Ali (peace be upon him) sent her to him to look at her, and he hugged her, and revealed her leg, is a story that is not acceptable, because it is hurts the image and the dignity of Imam Ali (peace be upon him), and the position of the caliph. So from my point of view, this story has no fact behind it, but it is the story of the storyteller.
Allah knows the truth