So we found out

• Shisui is Loulan
• Suirei and Loulan are half-sisters sharing Shishou as a father
• Shisui was Suirei’s original name, which Shenmei forbid her to use (because it bears the character of the “Shi” clan) and Loulan borrowed it when she needed a name for her alternate identity

I don’t understand why though Loulan’s name has no “Shi” in it, I thought it was important that she is marrying to the emperor as a representative of her clan. I originally thought Loulan was a title (something like the Duke of Edinburgh) but they seem to treat it as her name.