It is hard that it is like that but the things do actually belong to the other person? I have a stepmother and I am the youngest but my stepsister's children (my sister is 13 years older than I am, the eldest) got all of my stepmother's furniture when she died even though I grew up with it and was around it and used it the longest, even my desk from high school my stepmother gave to my stepsister when she moved into her house and it's still in her living room.

If I were you, I would practice expressing firmly what you want; if you are using the dresser/chest of drawers, tell them that (don't take the stuff out of them; when they come to move stuff you won't have it "ready"). As far as the desks, having a matching set is nice but it is just furniture; work something out with your sister if you can, just assume she was going to trade you another piece for it? Or, appeal to her sense of symmetry too, that the "pair" works well for you at this time and make a vague promise that you'll give her both in 10 years or something. I had a single half of an antique pair of soldiers and my stepsister's youngest daughter had the other, eventually, unasked, I gave her my half so she could bracket her fireplace.

If you are one person you can't really use two desks? Looks aren't everything? I think because you want them so much it feels like manipulation or poo-pooing your feelings (the "Looks aren't everything") but sometimes what other people say can be very hard for us to hear because we are trying to hold on to something too tightly.

I don't think they are trying to manipulate you because you have less, that does not make sense to me. I don't think they realize how much you want the items. We feel so strongly about some things we think everyone must notice but as strongly as we feel, there are people who don't feel strongly at all, to them it is just like furniture in a doll house, something to rearrange. You have a sense of possession they are not aware of. I would let them know how much you want those things but I would also look at your wanting to hold on to furniture/things so much?