If it's a game that was on here, I think it'd be kind of easy to sift for it by using some filters.
- It's an HTML game: goes from 18,727 down to 935 games
- Was it a male or female protagonist game? If male: 682 games
- Based on your description, it has incest: 174 games
That leaves you with 6 pages you'd have to look through at that point and you could just hover over to see whether any of them have the trace art-style you're talking about which takes like a second to check each one.
Then just click on the ones that look like they have trace art. After that, you can read the synopsis to see whether you remember it or search on the thread for "Abella Danger" to see whether anyone put a name to the character model used. Or check the dev notes to see whether the dev gives the names.
From there, there might be clues on anything like "sister's girlfriend" or something like that just from a cursory glance of the posts that mention "Abella Danger".
Then search on thread for "sister" to check and see whether anyone mentions that scenario from the game.
By the way, is it this? If not, then that's just the first thing I found with trace art after applying the filters. The game you're looking for could be further along.
Become Someone
(Edit: My bad, linked it to a post by someone instead of the main page. lol)