Of course there is a need to show financial support enough for the F2, as F2 cannot work. Clearly the grants etc the husband F1 is receiving is not enough to fund the wife's upkeep in the US as well. If that is the only reason for rejection then the stepbrother offering to fund the F2's stay should help.
Everest2020 seems to think the rejection was due to immigrant intent which is often a reason, but does not seem to be in this case as applicant was directly told it was a funding issue.
@khalan you should have been given a form when rejected stating the section under which you were rejected? Does it say 214b, or 221g. or something else?
As far as a "likelihood" of another rejection, a visa is never guarantted.