My brother would probably say the same about me because he'd find it difficult, but I'm not hard to please. I really apprecaite things with a bit of thought or meaning. What about a fave pic of your sisters kids/pets/partner whatever put on to canvas, or a framed meaningful poem. I was given a white rose called "Life begins at 40" for my 40th and whenever i see it i remember who gave me it. IF she likes her garden then i expect a plant of some description would be a welcome addition, or a big pot that she can put a specimen plant in. Gift vouchers are always great..amazon, crocus, etc etc, and so are vouchers for beauty treatments at a local salon or a spa if you are feeling more generous. A meal out or tickets to a concert, theatre, event. What about membership to the national trust or similar. Make some vouchers yourself redeemable for babysitting, car washing, lawn mowing, hoovering, decorating, ironing. Get her a case of nice wine, or a bottle of champers with some nice glasses. A fake fur throw, luxury bath products, a nice Radley purse, buy her a kitten.Take her out for the day