You don't say the ages of the children and I think that would be helpful to know.
I'm glad you can recognise the disparity among the children but it doesn't seem you are all that sympathetic to your step daughter's plight. I don't see you asking your in-laws to start including Maddie, your new step daughter, in any preparations for the future or gift giving. Maddie continues to be an outsider.
I think that's a mistake. Just as you said, it would be unfair to your children to have to give up gifts provided to them. But is is just as unfair to Maddie to not be seen as a blended part of the new family. If your in-laws can afford two grandchildren, they can afford three. They have may lost a son but now they gained a second granddaughter and should be recognized as such.
I wouldn't worry so much about the bedroom furniture at this point. But the clothes, now that IS a big deal. You commented on having Maddie share clothes with your son... Did you really think about that? To me, that shows your ambivalence to what Maddie has to go through with your family on a daily basis. No kid wants hand-me-downs and certainly not from a step sibling of the opposite sex, no matter the quality or brand name.
If Maddie is old enough to get a part time job, you could strike a deal for every dollar she saves for clothes you'll match it or even a 2 for 1 deal. There's lots of stores such as Plato's Closet that sells pre-worn items for teens and young adults and websites like Thread Up and Posh Mart that do the same thing.
Maddie feels like an outsider, even in her own home. Yes, that is going to build resentment and jealousy. So make it a point to exert more of an effort just being there for her, listening and helping her to achieve goals your children had handed to them.
You now have three children. It's up to you to make this work.