Don't feel guilty. Stepkids and society at large try to guilt stepparents by looking at some small aspect of a situation instead of the whole and they are basing it on antiquated ideas. 

Back in the day when the evil stepmother stereotype originated people got stepmoms when their mothers DIED. That is a vastly different situation from most modern blended families. In modern blended families kids tend to have step parents on both sides and still have both of their bios. 

If you restrict your view to only looking at what you are doing for your SD and for your BS then it appears that you're not being fair to her because you are buying something for your son but not for her. However putting blinders on to the rest of her family is not doing you justice. 

I would love to know what a step kid considered fair. I'm going to guess that for them to think that everything is fair they're going to need double of everything because both houses are going to have to treat them exactly like any other children. How is that fair if you have double what everybody else has? 

Do step kids really need four parents buying them things? Your bio son only has two parents so if you redirect some of your resources to your step kid you're taking away from him and he doesn't have a step parent on the other side to get extras from. 

Stepkids want to have the resources inheritances from all three or four parents. Your BS have 2 parents even if both stepparents are only contributing half of what a real parent does then the step kid is still getting the benefits of three parents. 

It depends if you just want to shut these kind of statements down or try to let her understand her logic of false equivalencies. 

To shut it down, my favorate is always....tough, life isn't fair.

If you want her to understand I might be tempted to ask her why it isn't fair. Perhaps point out that your son grows at a quiker rate than she does so needs new clothes more often, tell her that he also has to change outfits multiple times a day because of sick/pee/poop whatever.

If she still complains then I might say I am sorry that your Mom and Dad treat you like an "x" year old girl differently than your "y" year old brother. If you want to be treated like a "y" year old I suggest you talk to your Mom and Dad about it as they are your parents. 

I definitely wouldn't "hide" anything I do for my own children! If skid has a problem with you doing what every other mother on earth (including her own) does for their children, tough luck. I would just use one or two phrases - I'm his mom, I can buy him whatever I want. You have a mom to buy you things. Go ask her.

One of my biggest triggers is the expectation that SM should be ashamed or hide what comes naturally to them as mothers to their own children, or have to make it even for skids for some crazy reason.

My Skids used to make comments about the things I would buy and do for my BS. "Why does he get so many video games? Why does he have so much clothes? How many pairs of sneakers does he have anyway?"

They made the mistake of telling me that "Mom said she doesn't have any money and to ask dad."

To which I very quickly replied  "Well, BS is my priority. You need to ask your mom what she does with the money your father sends her if she isn't spending any of it on you, she gets PLENTY of money, enough to buy you WAY more stuff than I buy him." That shut those comments down real quick.

I don't bother to hide my preference for my children. They are of my blood and have some of my traits and I'm wholy responsible for them.

I have no responsibility for Adultskid and I don't want any.