Lothiriel was the youngest child in her family and had four older brothers. Elphir (who later when onto rule Dol Amroth), Erchirion, and Amrothos.
Lothiriel's father was Prince Imrahil, who fought alongside Eomer during the Battle of the Pelennor Fields, and went onto the Black Gates with Aragorn. It was Imrahil who discovered Eowyn was still alive on the battle field after her battle with the Witch-King.
Her aunt, Imrahil's sister, Finduilas married Denethor. So Boromir and Faramir are her first cousins.
A Dol Amroth legend states that all nobility has elvish blood in them. Mithrellas an Elf maiden, was among the elves from Lothlorien that accompanied Nimrodel when they were fleeing the evil in Moria. The elves strayed into the woods and were never heard from again. However, according to Dol Amroth legend, Mithrellas married Imrazor a Numenorean who dwelt in Dol Amroth. It is said that their son, Galdor, became the first prince.