I sympathize, since it can be so hard to find "the dress" for one wedding --and then to have to find another one! Plus sometimes we splurge and it can be hard to feel the investment can be recoup'd.
Also, I feel as though we should sometimes take a step back and remember that many folks may have one special dress or suit they wore for everything--weddings, funerals, Sunday church, you name it ! So I don't like to make having "new" dresses seem like a requirement for life.
Still, I think I would prefer to appear "different" in the family photos--sometimes that really makes mental connections for place and time--so would at least plan to start looking for a different dress if your budget allows, unless there's a fun family thing going on--you know, it could be possible for someone to be wearing her "signature" dress everywhere as a style statement.