If you have a reservation during EMH you don’t need a room key to enter just need to show them your reservation at the turnstile dedicated to EMH Character Dining guests.
If you want to enjoy anything else besides bfast with your aunt though, you may need to show a room key. It varies from day to day how strict they are about every person needing a room key to access EMH & most likely with one extra person would let you proceed but just know its policy to have one & they can very easily say no.
At the hotel they will print off a hotel key for every person staying in the room with their name on it. When my daughter as 2 & didn’t even need a ticket to get in she did need a room key with her name. It made for a nice, personalized souvenir after the trip, but it was needed for all of us to get into EE.
That being said, if there is capacity in your room, you can add your Aunt as a guest so they print her one as well if you wanted to enjoy any rides during EMH. Disney doesn’t verify who is actually staying in the room, as long as there is capacity they don’t ask any questions about why or who you are adding.
Lastly, and this is just my 2 cents, when you have EMH it is best used to ride rides with lower waits & better to save a sit down meal for once lines have started to fill up with people arriving so 9am at the earliest.