["I went shopping with my mom and sisters. My mom paid for my sisters' items but she did not pay for mine.", 'How come? Is your sister a minor and are you an adult?', 'My sisters are all adults, I am the middle child.', "That doesn't sound fair at all, I wonder why she did that.", 'I paid my bill and I felt so bad.', "If I were you, I would ask your mom why she paid for your sisters; items and not yours. That's so mean.", 'It is very mean. I have a good job so maybe they think I am better off financially.']
Now that I think of it, it most likely is the reason why; maybe she doesn't even think of it as a mean act and maybe means no harm by it. Regardless it is important to share how you're feeling with her.
My mom and sisters went shopping. My mom paid for my sisters' clothing but she didn't pay for mine.