QuiltE

As already said creams, but light creams, more towards white, as it looks like the blinds are white. Then pick up the pinks/peaches. Not brights, but keep the soft tones in the multiple shades of the floral print.

Chances are some greens may be in there, depending on what fabrics you find.
Also the tans/coffee of the same colour family as the chocolate in the drapes will help to coordinate it all together.

With the dark wood furniture and dark'ish green walls, there is enough dark, so the tendency is to steer clear of dark. Yet I would fear that if it is too light and airy, the sense may be that the large dark backboard and walls, are crashing in on the room. Does this make sense?

Because of this, I am thinking contrary to your star plans ....
I would consider a Trip Around the World or Boston Commons pattern or even a Blooming Nine Patch. Maybe even a Log Cabin.

For these, I would allow some of the the chocolate and green, to allow for the shadings from one to the other in the pattern of the quilt. But make sure they don't control the quilt, and turn it dark.

My thoughts are that these patterns could create a really well coordinated room, that feels warm, cozy and welcoming. Plus would leave lots of options, should your sister change furnishings, paint and/or draperies later on.