Ramayana is very well mentioned in Mahabharata as Ramopakhyan. It's mentioned as a conversation between Yuddhisthir and Rishi Markandey when Yuddhisthir lamented of his sad life and suffering of Draupadi when she was abducted by Jaydrath. Markandey then narrated him the life of Shri Ram. Mahabharat’s core story of Ramayana is very similar to Ramayana except few changes like Agnipariksha and Second Exile of Mata Sita are not mentioned at all. Mata Sita was extremely sad on the harsh words of Shri Ram towards her but she didn't give Agnipariksha instead of that all Devas came and gave testimony of her purity and then Shri Ram took his beloved wife back and then it's mentioned that he returned to Ayodhya and did Ashwamedha Yajnas as a King and everyone lived happily ever after. This can be considered an evidence of Uttar Kand being an interpolation as Mahabharata doesn't mention second exile of Mata Sita and probably Lava and Kusha were born in the palace only. It's possible Mata Sita being exiled again and Lava and Kusha born in Jungle is a historical fiction which always happens with famous personalities.
Yuddhisthir also in the epic never heard of Shri Ram before Markandey narrated him the story because Yuddhisthir asked about the Kula Shri Ram was born into but it's strange as in Raghuvansh Kusha was married to Kumudvati, sister of Naga King Kumud and Kumud is mentioned in Mahabharata as well. He is mentioned to be fifth descendant of Takshak Naga in Raghuvansh.
The story of Ravana's origins is also a little different in Mahabharata. Brahma had a Manasputra named Pulastya and he had a son named Vaishravan (Kuvera) with his wife Gau but Vaishravan served his grandfather instead of the father so Pulastya was enraged and through half of his body he made Vishrava. Vishrava didn't like Vaishravan and therefore he always wanted to take revenge but Brahma liked Vaishravan and therefore made him Lord of Wealth and he also made Kuvera revere Lord Mahadev, gave him Lanka and Pushpak Vimana. There is some discrepancy because Kuvera is called half brother of Ravana so a son of Vaishrava not Pulastya in this as well just like Valmiki Ramayana.
When Kuvera got to know about hatred of Pulastya and Vishrava for him, he gifted three Rakshasa girls to Vishrava: Pushpotaka, Raka and Malini.
Pushpotaka gave birth to Ravana and Kumbhkarna , Malini gave birth to Vibhishan and Raka gave birth to Khar and Shurpanakha.
So, the origins of Ravana and his siblings are entirely different in Mahabharata than Valmiki Ramayana. The Geeta Press English Translation do call Vibhishana as Half brother sometimes but in it Ravana and his siblings were born from Kaikesi but she is not mentioned in Mahabharata.
Manthara is called Gandharvi Dundhubhi who came to mortal world to enable the work of Lord Vishnu.
The story is very similar to Valmiki Ramayana with few changes here and there.