If you asked for birthdays, but said you couldn't afford presents, then explained why, I don't see how your nephew would think you were fishing. I agree with Bizi… people don't send cards and notes like they used to and they are appreciated more, but I think your nephew will think that was your intention about asking about the birthdays.

You only sent the email yesterday. Not everyone responds to emails immediately. He also may not know how to respond because he may feel badly for you and wants to choose the right words, especially if he is not going to extend financial help.

I think he will respond. Give it time.

However, I would not mention finances to him or your other nephew again. But I would ask your other nephew for his children's birthdays to be consistent on that point.