"Sister" doesn't have to mean his actual sister. Some people use sister in the some way "brother" is used. So, "sister" could just mean "girl". Although, knowing what Frank Black's like lyrically, he probably is talking about his sister.
To me "He took his sister from his head/And then painted her on the sheets/And then rolled her up in grass and trees/And they kissed 'till they were dead" means that he was imagining he was having sex with a girl in a grassy area - maybe a tropical forest? That's what I imagine - until they couldn't continue, and he ejaculated all over his sheets.
"Sister" doesn't have to mean his actual sister. Some people use sister in the some way "brother" is used. So, "sister" could just mean "girl". Although, knowing what Frank Black's like lyrically, he probably is talking about his sister.
To me "He took his sister from his head/And then painted her on the sheets/And then rolled her up in grass and trees/And they kissed 'till they were dead" means that he was imagining he was having sex with a girl in a grassy area - maybe a tropical forest? That's what I imagine - until they couldn't continue, and he ejaculated all over his sheets.