Getting too attached
 

This is my first post, so please excuse me if I sound a bit like a newbie.
I have always been best friends with a guy I've known when we were preschoolers, and as we grew up, our relation just got us closer and closer, to the point that his family feels like my own. He has a great relationship with mine but it isn't so deep.
At first it just felt like heaven, we had so much fun together. But later on, his young brother was born. I didn't know how to act since I'm the youngest myself (I only have an older brother who has really never paid attention to me), and during the first three years it felt weird spending some time around his place.
The twist came out of nowhere. When he was about 3-4, he suddenly started coming to me. I thought he only wanted to meet new people and play, but this has evolved into so much more. As time passed, he got attached, and so did I, which I really shouldn't. As of today, he's 6, I'm 18 and this feels way too weird to say the least. What began as something so stupid as taking care of him while my friend bought something for the three of us to eat, is now out of control. And it's a problem of both sides.
I imagined he would end up asking me to help him in some circumstances, but I never thought that every day I would stay till late in my friend's home, he would want me to put him to sleep. Sure, there's nothing wrong with that, but recently he even asked me to shower him. That really got me thinking if this is right or not. He has always been pretty shy in terms of changing clothes, and would never let anyone but her mother and my friend see him naked (my friend has a 19 year-old sister and a 15 year-old brother in addition to this kid).
And to make things worse, the family doesn't seem to care at all. They even ask me every now and then to take him to the cinema (which I do with pleasure) and to go pick him up at school. I've talked quite a few times to my friend about this, asking him to tell me his point of view, and he seems pretty happy, in fact, he has no problem in getting us together. Don't get me wrong here, I have gone from just knowing that kid to absolutely loving it, and I would do anything for him as if it were my best friend. But I can only feel wrong, think what's going to happen next and what should I do. If this was only my problem, I'd regulate my time with him and eventually stop seeing him (or drastically reduce our time together), but this is something he has done wrong without knowing. It's been too many times that he's told me he would love to have me as a brother, that he doesn't want me to leave, that he wants me to come to his birthday party etc.
Something else worth mentioning is that his mother has been under depression for quite some time now, and maybe that adds to his need for someone to play with, and since his sister is already busy with her degree, his older brother is tired of him and his other brother is now going through puberty and has never liked him...
So summing up, how can I stop this (if I should, although it would be really hard for me as well) or how can I make it easier? I really don't mind having him come to me as if I was part of his family, and neither do his parents and brothers. Thanks!

I'm sorry I not sure I quite get what you are asking.

Are you wanting advice on defining the constraints of your friend's brother or how to distance yourself from him?

Maybe try talking to the mother? Tell her you think the kid is missing time with her or playing with his brothers or something? Like, I kind of understand you want to distance yourself but at the same time what about the poor kid? Is he neglected? Will he be even more so neglected if you pull back how much time your spending with him? Like by rights the kid has a right to a decent home life, especially at such a young age. If you think this is lacking then I would speak to my friend (his brother) about it. I know when I grew up in my not so usual home I didnt think some things were wrong that others from normal backgrounds would. Maybe your friend doesnt realise that its not natural for this little boy with a big family to be so attached to you. Speak honestly with him and maybe start slowly pushing the little dude to the older brother. Like 3 of ye go to the cinema but then you head off before they go in, stuff like that. The kid prob associates you with all the good things in his life, getting brought out for treats, just hanging out with you without getting picked on by his older brothers. Depressed mothers tend to angry a lot too and kind of lack time with their kids like helping with homework, stories before bed. I think its good you are there for him really. Does he have many friends his own age?

Yes, I'm asking how to make this as easy as possible for everyone. Sorry I haven't answered back til now, I've been travelling and there are not so many places with Internet here.

His mother really doesn't care about this, she recently lost a baby before it was even born, so I'd say it's not the best moment to talk to her about this.
I've tried talking to my friend plenty of times, and as I mentioned earlier, he doesn't mind, even to the point of helping us make this bond stronger.
It kinda feels like what you're saying, last week he even talked to me about his middle brother "kicking" him (which I took with a grain of salt, after all young kids exaggerate things by a lot). Maybe the cinema thing helps, but then I also think what would go in his mind if I dissappear for no reason, and knowing he doesn't get much attention at home...
About his friends, he's got quite a few, but since its summer, he doesn't get to see them in 3 months or so, which makes him get even closer to me. And I repeat again, I have no problem with that, the real thing that's making me go mad is what's gonna happen to him after I move on.
Thanks guys for answering BTW!