I made a new account literally just to come in here and give what me and husband have been batting around here:
Stormfather claims there are 3 of them. One is away which the night mother has already said is slumbering. We know Dalinar is the first bondsmith as he is now bonded with the stormfather.
I felt pretty confident about the nightwatchers possible candidate but found myself here because I wasn't sure what the siblings could be. Lo and behold I wanted to say Navani makes sense for the sibling. So to offer my theory on the other possibility:
Night watcher- I noticed many mentioned Taravangian and I want to add one more thing: he wished for the night mother to give him the "capacity to save humanity". All things he has done has been, to his knowledge, for the greater good and not his power. He follows what he writes on his intelligent days. I think he is a strong candidate and is likely to seek her out again.