Rules in a shared kitchen can be negotiated when the issue is about preference. But when the issue is about health, particularly a gluten allergy, then we have a different conversation.

This is not about someone being controlling or overdramatic. It is about whether a person can safely use their own kitchen without getting sick every day. 

The gluten-allergic girl and her girlfriend are planning to move together and have already agreed on having a completely gluten-free kitchen. Suddenly, her sister asked to join them, but there was a simple rule she didn’t tolerate: keeping the kitchen gluten-free. She argues that if everyone pays the same rent, she should be able to use the kitchen however she wants. That is a good point, but not in this case. Paying rent doesn’t give a roommate the right to make the home unsafe for someone. And the renter gave her many options. She didn’t ask her sister to give up gluten. She only asked to keep gluten out of the shared kitchen. She even offered to set up a small kitchenette for her where she could have her own dishes. 

But the sister seems unwilling to accept her health needs behind the rule. At that point, letting her move in doesn’t sound like a good idea. If the sister resents the gluten-free kitchen before moving in, she will probably resent following the rules once she lives there. And by the way, if you’re moving in together with your girlfriend for the first time, I wouldn’t recommend adding another roommate to the new household. If you’re building a household together, letting someone else join you will affect your couple dynamics. Living together already changes the dynamics, and you’ll have to adapt to it, so why would you add something extra to make things more complicated? That’s a big no-no to me.