So far as background goes; Tashi was only child of his parents. So if they want sibling approach they need check both his grandparents for children (meaning he marries his aunt/uncle's child).<br/><br/>Also so far as I know in Naruto the genetics are not pinned down but some villages hold detailed medical info of the family trees to see and make calculations does the gene heritage or not. It was 95% sure measured Tashi gets red eyes somepoint but he was unusual late about it. But they did not expect anyway him to get true red eyes. Having such eyes is practically said miracle, a miracle which they would try to get.<br/><br/>Anyways Mibu clan would likely first check globally is there anywhere a another true red eye user and check is it;<br/><br/>A) Female<br/>B) Have children<br/>C) Siblings<br/><br/>Once that check up is done and if no result is found in grand search then they think other options. Only what is common with true red eyes is that each true red eye user has overall body excelency; meaning one monstrous stat (Str, Dex or Con) and rest excelent (+15 in range). So they would look physically very fit female shinobi very least and then think any potential heritaged genes. Also that candidate must have Trueblood human trait like Tashi and most clan have as they have been carefully breeding since start of clan to bring out top potential of each member. Anyways the normal Red Eyes has a very strong dominant gene so they could had married villagers more than once every three generations, more likely every or each second at least. Enrichen the gene pool and get varied talents.<br/><br/>But consider the sad fate what follows with having red eyes, the villagers are hesitant to accept marriage proposals from Mibu clan considering knowing their children may die in bloodlust driven madness while using red eyes. Worst case scenario live older age and driven constant madness state as red eyes causes stress to the mind (this part is my own fluff for the clan have kind of pride/sadness about having red eyes).<br/><br/>However we are not in Shinobi War period so there is not that great rush to get heirs married young but they likely try hurry the process much as possible. In shinobi war period he would not even get to do missions consider Tashi is the golden child of the clan :P. But good call none the less that he is lower level because his clan drags him down to super safe missions.