I feel like this is a pretty popular interpretation, but I always read this line like it's a bitter statement. Not saying there's physically no room for Sunny, but there's no room for someone like him.
Also I didn't want to use up an image slot for this, but there was a bit of flavor text in regards to Hero and Kel's family photo.
>"KEL AND HERO's family photo. What a good-looking family..."
It reminds me of an unused bit of flavor text from the 2019 build that isn't used here because you can only open the fridge, not examine it.
>"KEL's fridge. There's a bunch of colorful magnets on it. What a fun-loving family..."
I don't think Sunny would ever admit it even to himself, but I think deep down he might feel a little bitter about how happy his neighbors have been while his situation has been decidedly not okay. 'Everything Sunny needs, Kel has' and all that. Or maybe the "..." is being used similarly to how it's being used in "Wow..." and I'm just reading spite and bitterness where there isn't any.
And while it's in recent memory, Kel's Mom telling Sunny he'd always have a home with them was emotionally hitting on repeat playthroughs. Knowing how things go and what the twist is, it makes what she says so tragic. He might never be able to go back.