I've worked for a brief period with family before and it can quickly become a complicated situation.
A few words of wisdom from my experience:
1) Establish what your role is and what aspects of the company you control:
Just because it's your family, doesn't mean you'll suddenly get a lot of control. When I was brought on as a Operations Manager I thought I had a lot of freedom to control how things operated. However, I quickly realized that my parents and their business partners still saw me as an employee rather than a strategic partner. Making this clear up front and observe how they react when you offer strategic solutions. Do they brush you off or do they act on it?
2) What are realistic contributions you can make to the company and do they align with your goals?
Your comments about your parent's hesitancy to put additional money into the company means you'll have to fight to hire a developer and update the core product. If you lose this fight, what other opportunities do you see in the company? If you don't see any, then it's probably better to move on and find a project where you have either more control, or more options.
Echo the idea about the landing page, but I wouldn't spend the money on adwords. You've got a lot of resources through forums and friends to push your idea and a ton of blogs to reach out to.
Peldi from Balsamiq has a great primer on how to market a new start-up on his blog titled Startup Marketing Advice. Read. Implement. Learn. Rinse. Repeat.
From a marketing standpoint, I wonder how you actually find the customers you're looking for. Most start-ups seem to generate enough interest through HN, TC, and other niche focused sites. However, I'm not sure these are your key audience.
From my own shopping experience, I generally find one brand of shirt/pant/shoe that work well and then it's my go-to brand. Anytime I need to buy something new, that's where I go. I hardly deviate or try something new because as you mentioned, I hate shopping and as other people have mentioned, fit and feel are important.
However, I think you can expand your product by saying "hey, if you like Shirt A, then maybe you'll like Shirt B". After all, the style from one brand often remains the same and even guys will want different types of clothes for different events/moods/etc.