Yisro is saying that if Moshe is not in a position to take care of his family (which is why he had sent them back from Mitzrayim) then Yisro would still continue to take care of them as members of his family. Moshe is currently responsible for the entire nation and Yisro understands what is involved in that.
Rav Hirsch explains in Yisro 18:5
In V.2 where we are informed of Jethro's decision, and in V.6, in the
message with which he announces his arrival to Moses, the children are
called בניה Zipporah's children. Jethro wishes to be neither a burden
nor a hindrance to Moses. Moses had sent his wife and children home to
him. It could very well be that Moses was still not in a position to
keep them with him. Accordingly, Jethro had the intention of still
keeping his daughter and grandchildren with him. It was not to take
them back to his son-in-law, that led him to go with his daughter and
grandchildren into the wilderness. But here in verse 5, the objective
fact is recorded of what they were to Moses, and of course the fact
was, that they were Moses' father-in-law, his sons and his wife. That
is why here the sons are mentioned before the wife, who in any case,
as the father-in-law is mentioned first, comes last as being dearest
and nearest.
