And though I can't write stories I can sometimes come up with a plot summary. So here's what I came up with as a MS plot. You'll have to add the pretty words yourselves.
Mark's sister, Mary Sue, comes over to NY for a visit. She is a ravishing beauty and fiendishly intelligent. Illya falls for her (Napoleon makes a peripheral appearance in the story to decide she's too good for him and leaves her to Illya) and takes her out to dinner and the theatre. But nothing beyond hand holding and gentle kissing because she's a respectable girl.
Illya tells her about his childhood in a Russian orphanage, which he has not confided to anyone else before, and shows her the medallion which is the only memento he has of his lost family. She then returns to her work in Europe as her holiday is over. Illya is heart broken but realises it has to be. Mr Waverley would like to ask her to join UNCLE but knows that her work is more important than theirs (she's going to eliminate world poverty or find a cure for cancer).
Mary Sue thought she had seen a medallion like Illya's before, but doesn't say anything to him about it. She wants to confirm her suspicions first, before she says anything. Back in Europe she goes to see her colleague who has a similar medallion and gets him to talk about his family. When he was small his family were on a diplomatic mission to Russia, but were caught up in one of Stalin's purges. The family managed to flee the country back to their native Sweden but in the chaos the baby was lost and left behind. They never managed to find the baby or learn what had happened to him, and their mother still mourned his loss.
Mary Sue arranges for the family to go to NY where she reunites them all. "Oh, Mary Sue, how can we ever thank you." Illya now becomes Swedish (a much better nationality than Russian, cos nobody likes Russians)