My sister-in-law has found a house in their neighborhood that has been vacant for years, and lo and behold, the owner has passed away and the heirs are looking to sell.

Many realtors are not going to work in low value asset areas as those are generally rougher neighborhoods that are probably deemed to unsafe especially for a single women agent to be out there.

When I looked at prices recently they look like they've gone up quite a bit but I know they vary a lot depending on the neighborhood. 

It's a small place so I'd guess it won't rent for enough to cover your expenses anyway so I'd probably sell, especially if the loss is small, it unless it's in a great neighborhood that you think will do well over time. 

I went out with a realtor friend and searched neighborhoods I was familiar with.

Now this property is in a class C neighborhood with a calculated 11% cap rate estimated.

I went out with a realtor friend and searched neighborhoods I was familiar with.

If they Wanted to stay inside a duplex within class A+ neighborhood?

Just no guarantee if things get better or worse in your neighborhood.   

For more info on Cleveland you'll want to read The Ultimate Guide to Grading Cleveland Neighborhoods.