S.D.
As a wedding planner (and a SIL myself). I recommend 'old, borrowed and blue'. The 'new' should be the dress. find something of yours, something quite personal (preferably old and blue) to lend her on her wedding day. An old standard is Grandma's hankey. Jewlery is a nice option. Present it to her in private and let her know how important the piece is and how important she is to you. That should help
Good luck