Hope it helps. You aren't alone, but your situation is certainly complicated by being in someones house though, your brother doesn't sound very sympathetic (wait till he gets his own kids - you will have your revenge! 
)
Still if you can get his food intake up in the day the night problem should go away, or at least improve.
I think sometimes children feel very 'fed at' (the equivalent of being 'talked at' as opposed to 'talked to') and I find my ds can be more interested if he has some control, so if I am feeding him ( he isn't that great with his cutlery yet, ho hum, another thread) I let him feed me and we take turns. He usually eats meals he has 'helped' cook better than meals which arrive out of the blue, though again that could be trickier in someone elses house perhaps.
My DS nursery have started doing soups! Not something I expected him to like but they give a little bit and let them dunk the bread and eat it that way, that's another novelty idea.
I use squeezy honey to draw pictures in his porridge and he has to eat the picture.
I have a plate with rabbits on and if he eats another piece he can see more rabbit.
Mine is terrible at eating veg so I try to get his 5 a day in via fruit which is easier at this time a year as he can eat his own body weight in strawberries and they are good for food on the go. But also easily peeled satsumas he can have a go at himself (can you make an elephant shape from the peel iykwim) and nice ripe peaches (this peach is furry! shall we stroke it before we eat it? How did that stone get in there!? etc)
He has gone right off bananas (contrary monkey) so I make banana milkshake (1 x banana + glass of milk whizzed up in whizzer) is a huge hit even though he doesn't like banana 
If I'm feeling generous or want to fill him up more I add strawberry yoghurt or even some hot chocolate powder to make it pink or choc coloured.
I did try saying to ds that if he eats so-and-so he can have such-and-such but found as soon as he realised the nice thing was available he would push his plate away and just want that - no concept of deferred reward yet! So I've stopped doing that and just try to keep food interesting and involving.
I also use soreen (ready sliced) as it is high energy and healthy (though need to clean his teeth after as quite sticky) he loves it and I don't mind him having it.
I shop in asda and they sell Urban Fruit in the fresh fruit aisle even though it is dried, but the great thing about it is that it is the pure fruit with no additives at all but have the texture of a chewy sweet, so win-win. www.urbanfruit.co.uk/ Mine loves the mango or the strawberry.
Good luck, hope things improve and you get your own place soon.