While scrolling through the Wikipedia page of Lord Kubera, I saw that it was written Kubera's son's wife was Rambha who was raped by Ravana. So, what is this story? Also, His sons were cursed into two trees, who were then liberated by Lord Krishna. Can someone explain this too?
The first story can be found in Valmiki Ramayana: Uttara Kand: Sarga 31. After that incident, Kubera's son Nalakuvera cursed Ravana:
[Nalakuvera said:] O fair one [Rambha], since thou hast been ravished by him [Ravana] despite thy unwillingness he shall never be able to get by an unwilling damsel. Whenever he shall, stricken by lust, ravish a reluctant damsel, his head shall be sundered into seven pieces.
The second story can be found in Śrīmad-Bhāgavatam (Bhāgavata Purāṇa): Skandha 10: Chapter 10:
one day they [Nalakūvara and Maṇigrīva] were enjoying with naked girls in a lake and shamelessly walking here and there. Suddenly Nārada Muni passed by, but they were so maddened by their wealth and false prestige that even though they saw Nārada Muni present, they remained naked and were not even ashamed.
Due to this, Narada Muni cursed them to become trees for 100 Deva years.
Later on Lord Krishna librated these trees:
By dragging behind Him with great force the wooden mortar tied to His belly, the boy Kṛṣṇa uprooted the two trees. By the great strength of the Supreme Person, the two trees, with their trunks, leaves and branches, trembled severely and fell to the ground with a great crash.