Mother in law to be will not tell me what she is wearing to my wedding?

She just told me she had bought her dress and when I asked what it was like said that she wouldnt show me or anyone else and that she wanted it to be a suprise on the day. Is this a bit strange? Im not a bossy or fussy bride but have been reading that usually the bride is involved with what themother of the groom and mother of the bride wears