What a pretty house and I agree with colour. Teals would be spilling in this area if me but if your mother in law likes nuteruals use the nuteruals or some gold, yellow tones may please her. There is a linen colour bear makes that is soft and gives a little colour but is not overwhelming it would also go with the railing and the grey floor. I think anything darker in the beiges would make the railing looking blah. You could though if she liked it paint the railing and I'd def. paint the trim work white. I like the wood trims but in this case I think the white would look better. Or maybe a soft mural going up the stiars in some nuetral colours more like what they would had used in the 1920-30's. That way it won't be overwhelming to her and that could be made to be very appealing. Then paint the door a nice colour to compilment.