This is not currently possible.
At present there is NO, Flexbox, CSs-Grid or any other layout method or algorithm that allows for centering an element without taking the size of sibling elements into consideration.
You will require Javascript to adjust margins or re-arrange your HTML structure to accomodate your design choices.
It IS possible to center the specific element using absolute positioning (although you have ruled this out) but this ignores the other siblings completely thus their "positions" might be affected and we again fall back on JS being required.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…