How to force a selection in one field if another webform field is a certain value
You could use normal conditional fields for that. Simply create two "How many t-shirts" fields, one with only one enabled option, second with both available. Then, make them visible depending on value in "Membership type". It is possible since Webform version 7.x-4.0-alpha2, and earlier using this module.

Sure, this will create two separate entries in results, and only one filled, but if your traffic is low, it will be easy to read. And if it's high, you can aggregate easily in your warehouse handling software. Or in spreadsheet of your choice, after export.