You could ask hubby or FIL to suggest a voucher "Mum she's really fussy about what wools she likes, I wouldn't buy her wool myself, I would get her a voucher, she'll love you for it and definitely get something she really really wants that way... when you walk into the shop you will understand, it's like a warehouse full of all different stuff, sock wool, cotton, acrylic, differnt fibres and weights and colours, superwash, I am always lost but I get her vouchers".
Or next time she hints, you could say 'ooh that's SO thoughtful! Anything handpainted would be great/anything sock weight/I really love this stuff called Noro/as long as it's blue or green/shall I write it down? that's K-O-I-G-U'
If she is hinting like that maybe she wants a hint about what to get!