I think it depends entirely on your relationship with your Mother inlaw. For mine I would not charge anything but expect her to buy the fabric and threads hooks etc. Same goes for close friends and sisters etc, however having been a curtain maker as part of my job I can vouch that there can be a lot of work involved in say, lined curtains with tiebacks and valances etc!
What kind of curtain are you making, long or short, is it a simple unlined rod pocket type or the more elaborate as I have already mentioned? I think you need to take those things into consideration perhaps.
Gal