<human>: My gf and I have been together for 18 months and things are going pretty well. There are two things I'm concerned about for the future though. 

She is one of two children with a younger sister, about 10 years younger. This girl is severely developmentally disabled though, she will never be on her own. Her parents are older and have a history of health problems. 

It had been mentioned multiple times that she would inherit care of her sister when her parents pass away. If I plan on staying with this girl, it means that I will likely inherit care of this girl.

Now I'm terrified of having a disabled child, and would likely advocate to terminate a pregnancy that showed severe genetic disability. 

I don't want to take care of this kid. I know it sounds horrible, but I see what it does to my gfs parents and how stressed they are, I don't want to start my life and know that sooner or later we would never be able to travel by ourselves, or leave her under someone else's care. 

I've asked my gf before and she says she wouldn't put her in care or anything, she would want to take care of her.

What do I do? Am I an asshole to break up with her about this? Do I try to convince her to place her sister in nursing care eventually?

Can you summarize the given text in a few sentences?
<bot>: my gf had a developmentally disabled sister we would eventually inherit if we got married, and I don't want that burden at all.