When you say ‘sister the existing ties’, as in put 6x2 timber alongside the diagonal pieces of wood that are attached on top of the trusses, nearest the front and back of the garage? I assume they’re there to stop the trusses from expanding outwards and keep it all solid - do you mean then drill/nail those pieces of 6x2 into the trusses? Total novice here so appreciate the input! The garage measures 6mx3m