I think this is getting a bit too specialist.
Slipcovers and curtains are usually made by specialised tailors/ seamstresses who are NOT upholsterers. Upholstery and furniture recovering is the work of a ταπετσέρης and I have not seen many women do this job, but if there are any then they are probably ταπετσέρισσες or something like that.
People who deal with ραφή κουρτινών και καλυμμάτων επίπλων are usually μοδίστρες. Remember that in Greece, unlike say the UK, curtains are not used for insulation or blackout, and there is no fashion for the elaborate, highly decorated constructions that are typical of English country cottage style, so curtain making is not as specialised as it is in the UK.
By the way, μοδίστρα is not colloquial. It is the proper term for a woman who makes women's clothes. Men are not involved in this trade for reasons obvious to anyone who has ever had a fitting.
Ράφτης is a man who makes men's clothes (or as my grandmother would call them sometimes φραγκοράφτης, a man who makes western style men's clothes, aka trousers). Women who make men's clothes are ράφτρες (but usually the women who worked in that trade were assistants doing the stitching, and I doubt they would do fittings or measuring).
If you are looking for someone to make you a pair of curtains, it is safer to ask ξέρετε κάποιον που να φτιάχνει κουρτίνες;