I think labels in general can be very freeing and very restrictive at the same time,
for instance I have a bisexual orientation of being Queer,I dont use the other labels Pansexual, Bisexual, etc because they jut never clicked for ME. If you want to be called a sissy go ahead, if you don't like the label and maybe feel it doesn't suit you in the capacity you think a label should (an identifier) then just be you, sissies are all varied also and don't all have to be the same. Just as abdls who are "boys" that like girly clothes don't have to use the label sissy.
If someone wants to know what you like wearing just tell them, purely what you are about. When someone doesn't get my queer identity I explain and thats the end of the convo, You dont owe anyone anything.
Now if you become comfortable with the word sissy, and wanna use it go for it, but dont feel pressure to though. You can always change your mind nothing is set in stone.
Hope this helps