But what can I do today to these my daughters, or to their children whom they have borne?
Laban uses a question to emphasize that there is nothing he can do. This rhetorical question can be translated as a statement. AT: "But, there is nothing I can do to bring my daughters and grandchildren back with me." (See: rc://en/ta/man/translate/figs-rquestion)
let it be for a witness
Here the word "witness" does not refer to a person, but it is used figuratively and refers to the covenant that Jacob and Laban are making. The covenant is spoken of as if it were a person who is there when they agree to act peacefully to one another. (See: rc://en/ta/man/translate/figs-personification)