In fuzzylite, you have a Function membership function, which takes any string and converts it into a binary expression tree and evaluates the expression at runtime. You can have any relationship between variables you want. You can check it out in QtFuzzyLite 5.
you are going to need to provide more information about your requirements. Could you please provide a fuzzylite engine and present what you wish to achieve?
You can create any number of linguistic variables you want, and refer to their values from any other variable in a Function membership function using any number of mathematical operations, but I am not sure if this is what you are aiming for. I would need more information to help you.