It is very common in NZ for people to donate money to charity's or good causes rather than spend money on flowers, usually the family make the request as that is what the person who died would have wanted, a trust fund for the remaining twin is not the norm here, but however I see nothing wrong with it, better to help the living IMO, so go with your feelings.

P.S maybe there is something you do not know, just a thought, maybe the remaining twin has health issues, or something along those lines, I still think it is better to put money into the trust fund if that is what the family want , than to waste it on flowers they donot want,JMHO.