Can an untreated brain tumor cause a brain aneurysm?

Unlikely: These are two separate medical conditions with a little chance to overlap. However brain hemorrhage can occur due to either of them. That is the only overlap i can think of.

Not usually: Strokes and brain issues can come with either but they are not very common together.

Unlikely: These are very different entities. However, there is the rare instance of a slow growing benign tumor such as a meningioma that may invade or wrap around a major blood vessel. However, if an aneurysm forms it is not like the typical saccular(berri) aneurysm.