Can a protaganist (or character) who engages in incestous relations be likeable? Yes, of course! Anyone who truely thinks they can't probably wouldn't be anywhere near an incest game.
As people have pointed out though, being likeable isn't the same as being a good person. Bad guys and villians can also be likeable. But I assume what you really were asking is if they could be a good person who the player empathizes with and supports. The answer is still yes, but they have to be a well written charatcer. Which means they can't be a blank, faceless dick, there just to fuck chicks. That means character development and all those other dificult writery things.
It also means your character can't really do any shady stuff. No rape when they're awake, no rape when they're alseep. No sexual assault, aware or otherwise. No spying, no drugging, no mind control. No third party alterations of perception/feelings/reality. And two things that likely stop a lot of devs feeling it would be a worthwhile endeavour: A single LI and ROMANCE. A character who is just whimsically screwing his mother, auntie, older sister, twin, younger sister, cousin, neighbour, neighbour's mother, neighbour's sibling, best friend's mother, childhood friend, boss, librarian, local shopkeeper and almost any vaguely attractive girl he comes into contact with, ALL AT THE SAME TIME, can't really be all that likeable. He doesn't have time to be. Even worse if he's telling more than one, or all of them, that he's in love with them.
In short, a likeable incest protag is possible if he's firstly a likeable person. The incest is honestly superficial.