My view would be - while they are young, give them many chances, because many selfish teens grow into thoughtful adults - and while many don't - you won't know which way your cousins will go until later.
In the meantime, they may learn something through your good example.
Of course, if you chose not to pursue a relationship with your cousins, no one would blame you.
And of course, you could try something like, the next time they call for help - help them out, and then ask them "Hey bro, may I run something by you? (answer - sure, what). (You answer) You know I'll always be here for you when you need help, and I think of you like a brother. Something's been bothering me - last year I was in hospital, and you didn't come and visit me. May I ask you why you didn't visit?
The point of such a conversation is manyfold :
- you ask permission so that he can't feel that you ambushed him with your question
- you are telling him that you think of him as a brother
- you are telling him that his not visiting bothered you
- you are asking him to let you know his reasons for not visiting you
- and through inference, you are implying that you believe brothers visit brothers in hospital - that doing so means something to you, etc (of course he may be too young to understand that, but it may resonate with him at a later time if you go your separate ways)
...and no where in there is pre-judgement