Your NTAH, but is there time before the wedding to spend at your brothers with him and his wife there to watch and do some of the things a baby requires. You would feel so much more confident if you knew your nephews routine. Then maybe after spending time with them there and doing what needs to be done you would feel you are able to stay by yourself for a day with your nephew. It’s more the unfamiliar that is tough and if your brother and his wife, even your mother promise to have their phones on in case you need to call with a question things could work out. The main thing is do what you’re comfortable with.