It's quite normal for relatives to wear buttonholes. At my brothers wedding myself, Mr Spudqueen and my Aunt all wore buttonholes. If you feel comfortable wearing one, and would like to wear one then by all means do so. If, however you would feel subconscious, or don't want to wear one, then don't. To be honest I don't think that anyone will be that bothered either way, there's enough going on at a wedding without wondering who has or hasn't got a buttonhole!
If you do fancy wearing one though you could pop into your local florist and ask them to do you a little spray to match your outfit. These can be of any flower and are nicer than a buttonhole, especially for a lady.