Most Myanmarese girls don't use their father's last name. But they usually use their ethnic name. If they are Shan ( Tai-speaking peoples ) including Thai, they put these words below at the front of their name.
Nan, Nan Kham, Nang, Kham.
If Yoon and her sister come from Inn Lay.
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They may be Shan People. Shan people are very close to Thai and their Language is 90% similar to Tai/ Thai Language.
That is why I would prefer Khun Ni Ko. Khun comes from Thai and Ni and Ko are still Myanmar names.
Thinzar Bo meaning - Thinzar is of Indian origin and it means "A Buddhist Girl".(That's why I don't want to use it in adult visual novel) Bo means Leader/ fighter and Bo Kya means something like Bully.
But in Myanmar, most girls with Thinzar are hot as far as I see on the internet.
You suggested Cherry Ni Ko and I like it but I think Cherry Mi Mi or Cherry Ko would be better IMO. Because Ni means red and Cherry is not red.