My [28M] girlfriend's [26F] family will invite me out to dinner, and then expects me to pay for the meal. How do I get out of this situation?

Using a throwaway because she knows my main.
My girlfriend Casey and I have been together for a little over year. Things have been going really well so far, this is the happiest relationship I've ever been in.
Casey is really close with her family, and they live only 30 minutes away, so we see them fairly often. Casey's family consists of her mom [50sF], and her two sisters (24F and 22F). Her sisters live together in a neighboring town. Casey's dad left when she was 4 years old, shortly after her youngest sister was born.
I really love Casey's sisters, and I genuinely enjoy spending time with them. Casey's mom is kind but can be annoying or needy at times. She talks a lot about being a single mother and raising her girls on her own. Since Casey's mom was a single mom with three kids, they grew up incredibly poor. Casey's mom doesn't have a stable job and hasn't for a long time.
I do well for myself financially. I'm a software engineer and Casey works at a nonprofit. I make more than enough money for the two of us, and Casey is really passionate about the work she does (even if she doesn't make much money). I've never had a problem taking Casey out to dinner, or covering for things if she's short on cash.
When Casey and I first started dating, we went out to dinner with her mom and sisters. In an attempt to make a good first impression, I offered to foot the dinner bill for everyone. Casey's mom and sisters were really appreciative, and I remember her mom said something like "It's nice to have a man around again". It made me happy to cover dinner and to know it was so appreciated.
The second time we went out to dinner with Casey's family, her mom invited us out. At the end of the dinner, the bill came and just kind of sat on the table. I took out my card to pay for our portion of the bill, and Casey's mom thanked me for covering the bill again. I was kind of thrown off by this assumption — but I also didn't really know what to do in this situation. So, I covered the bill again.
A little over a year into our relationship, Casey's mom expects me to pay for the bill whenever we go out to dinner. It's gotten to the point where I will decline dinner invitations from Casey's mom, because I don't want to cover the bill.
I've talked to Casey about it, and she's told me that she understands completely — and apparently talked to her mom about it. But in the few instances that I agree to go out to dinner, the bill will just sit on the table until I grab my wallet and get thanked for picking up the bill before I've even offered to pick it up.
How do I dig myself out of this hole? I've thought about just not reaching for my wallet when the bill comes. I also don't know of a graceful way to say "I'm not covering the bill" when Casey's mom thanks me for covering it. Help!
TL;DR - Girlfriend's mom expects me to cover the dinner bill after I offered to foot the bill one time. I don't know how to get out of this situation without making things awkward.


Source.

Someone wake me when this chap has finished his bleating.....

Go DUTCH :))