I don't want to be worried, but....

He told me how he wants to create a bond with a girl (someone his age) to create a brother/sister relationship since he doesn't like his current sister and how she is, and she doesn't make much of an effort to bond with him. I could understand why, but for some reason it troubles me? I know it shouldn't, but the way he  talked about it made me worry. He said he wanted to create a bond with a girl, become close and do things a brother and sister would usually do. I feel like a relationship like that, it would be a suspicious? I'm afraid that feelings will develop if he were to do this, or they would do sister/brother things that are too close and would be put in situations with her that would be awkward for me. 

He says he won't do it though since he understands 100% how I see things and why someone would worry about this and situations that would happen, but it makes me feel guilty that he won't have that sister figure he always wanted and that I'm the reason he's not, and he's been thinking and talking about this for some time as well... What do I do? I feel like I'm so wrong in this situation.