Theft can be a serious sin, but there are potential mitigating factors to consider. This might include the amount (stealing a paper clip from a neighbor’s desk is probably not a serious sin), the reason (stealing bread to stay alive is probably not a serious sin), etc. Intent to make restitution would probably be necessary to keep even legitimate mitigating factors from making the sin serious.
The amount is probably trivial enough, but there are other factors here in this hypothetical crime. We have here a betrayal - the brother was explicitly told not to take the money but did so anyways - for illegitimate reasons. Trust is irrevocably lost. Furthermore, you say that he “takes the money openly” which is, in effect, robbery. This seriously violates the dignity of the victim, that might makes right.
I can easily see the sibling’s concupiscence in putting love for a created thing over the love a fellow Christian (and a family member to boot) as the first in several manifest sins that could lead eventually to Hell. This is indeed serious; nip it in the bud if you can.
