Baby Advice!!

Guinevere
Okay, so I have a 15 month old little half sister. (She's perfect btw lol). Anyway, her mom has some health problems, so every now and then I'm sort of like an auxiliary mom to her.  
Recently, she and I have really bonded. And I mean REALLY bonded. Like she's miserable when I leave, she always has to be in my lap (unless she's playing of course), and since she's been sick, she insists that I carry her everywhere.  It's getting a little hard because depending on the night, she acts as if I was her actual mother. And those times are becoming more and more frequent. And i feel terrible because anytime I have homework to do, I feel like I'm abandoning her.  And honestly I LOVE how close we are, but I don't want to put her mom in an awkward position either. 
Not to mention the fact that, I'm infamous for having baby lust lol!!! And I would LOVE to have my own, but there's a reason why we haven't yet lol. I mean I have work and college. And I know there are mothers out there who do it, and my god, do I look up to them; but honestly, I'm not sure I could.... Although I'm started to have to!
 What should I do?  Is there such a thing as being too closely bonded?  I'm way out of my depth here.  Any advice would be greatly appreciated!