Eligibility for EU citizenship? (Long but has a summary)?

My brother has recently been thinking about working in the EU, and this would be a lot easier if he had EU citizenship. This also got me curious about my own possible eligibility, and, since I'm absolutely awful at finding stuff on the internet, I figured I might as well post a question about it.

Family History:

My mother's mother (still alive) was born in Gottschee, Yugoslavia (modern day Slovenia) in 1920 to an Austrian man and a Gottscheer (ethnic German living in Yugoslavia) woman. My grandmother was a Yugoslavian citizen. They left in 1927 to go to the USA, but my grandmother's Yugoslavian citizenship was never formally renounced. All Gottscheers were exiled in 1946.

My mother's father (deceased) in 1920 was born in the United States to a Polish family and was a dual citizen. They moved back to Poland immediately after he was born, and my grandfather was raised there. He and the family left in 1927, and returned to the US. (If you're wondering, my grandfather was basically an anchor baby.)

Language Requirements:

We both speak German proficiently, and I speak Gottscheerish passably. Neither of us can speak Slovene or Polish, but we have the means to learn the languages.

Summary:

My mother's parents were Yugoslavian (Slovenia), American and Polish citizens, and my great-grand father was an Austrian citizen. We are of Gottscheer, Austrian and Polish descent. My mother is NOT a dual-citizen. We are American citizens. We have documentation to verify all of this.

We're wondering if it's somehow possible to acquire citizenship of any of these countries (Slovenia, Austria, Poland) based on descent?

Thanks for reading.