The question is derived from Suigetsu's dialog to Mitsuki in Boruto: Naruto Next Generations episode 30.
Boruto is your sun, isn't he?
We both see the sun and the moon has been associated with Hagoromo and Hamura Otsutsuki. They can be reincarnated since both are dead, and Naruto and Sasuke are reincarnations of Indra and Asura respectively.
Is this possible?
Boruto is your sun, isn't he?
This dialogue is a figure of speech. Here Boruto is Mitsuki's sun, meaning, he brightens up Mitsuki's day. Or to put it simply, Boruto brings joy to him. The context of the scene is crucial.
Next, the Otsutsuki brothers do not reincarnate. Indra reincarnated to fulfill his mission of a perfect world, under his reign. Ashura reincarnated to stop Indra and save him from the blindness, with his love. Hagaromo entrusted Ashura with Ninshu, thereby clearing all his goals. Whereas Hamura entrusted his will to his clan. Both had no reasons to be reincarnated.
Here's a much more detailed answer from Reddit (credits to @Wondercricket):
In Naruto Gaiden: The Road Illuminated by the Full Moon we get to see a bit more of Mitsuki's backstory and how he is related to Orochimaru, as well as how he is different from Orochimaru. Orochimaru runs the same test on Mitsuki several times, wiping his memory each time it fails. The test revolves around Mitsuki gaining 2 options, if he chooses either, he fails. He only succeeds upon finding a 3rd option for himself.
When he does this, he is rewarded with information about Boruto. Whom he, at that time, decides that Boruto is his "sun". His "light" in the darkness that will shine a bright future for him if he is to follow him. Hence, he holds respect for Boruto as well as hope.
