Should I feel bad?

So I watch my husbands 10 month old niece a few times a week. I work full time but when I'm off his sister asked if I could watch her and I have no problem with it. My sister saw me post a picture and past a comment about me watching her. My sister lives about 45 minutes away and I think she's mad that I don't watch my 15 month old nephew. I'm his "nanny" / God mother. I love him to death and would watch him any time of day but she has him to where he screams when she's not around and he will go on for hours. Also she never asked me to watch him and I can't go out there everyday to do it because it's too far of a drive to make everyday. Should I feel bad for this? I go see him once or twice a month, but I'm really feeling guilty that I'm doing my sister in law a favor and watching her daughter and not helping my own sister out. I'd watch him in a heartbeat if she could bring him here, even with the separation anxiety. My husband keeps telling me not to feel bad because she didn't ask and he knows we don't have gas money for me to drive out there a few times a week. Please help because I'm crying and losing sleep over this guilty feeling.