Am I right that my mom shouldn't be keeping this from my sister?

OK so, my family is kinda strapped for cash (My dad has a job, but it’s a night job and the pay is pretty lousy), so my sister and I might not go to camp this year. This would be the second year for her (she’s 9), and the 4th year for me (I’m 13). But my sister has no idea that we might not go, and my mom isn’t telling her. My sister says “I can’t wait” about 5 times a day, and she talks about last year all the time and how awesome this year will be. I just shrug it off and don’t let her know that we might not go, because if my mom isn’t telling her, I’m not going to take it into my own hands. But I just don’t want to see her disappointed! This is making me feel terrible that there’s something I could be fixing but I’m not that will make my sister disappointed. So what should I do? Is my mom right to not tell her? Why would she be doing that?

P.S. It’s a one-week camp, not all summer, and we might be able to get a ride for at least one of us both ways (we’re going at different times).