Could someone please help me with Spanish homework?

I am busting my brain over this. I read my Spanish textbook about this, but I still don’t understand how the last names in Spanish speaking countries work. Could anyone help me, please? Here is the picture: http://i419.photobucket.com/albums/pp277/lindsmith88/3-1.gif

La familia Vargas:
1. Maria Teresa Vargas Casona
2. Felipe Vargas Nunez
3. Alejandra Casona de Vargas
4. Gustavo Vargas Casona
5. Julia Guevara de Vargas
6. Rosita Vargas Guevara

La familia Garcia:
7. Joaquin Garcia Munoz
8. Hector Garcia Ramirez
9. Elvira Munoz de Garcia
10. Claudia Garcia de Moreno
11. Roberto Moreno Alvarez
12. Claudio Moreno Garcia

Felipe Vargas Nunez (#2) es _____ de Maria Teresa (#1) y Gustavo (#4). Es _____ de Alejandra Casona (#3). Es _____ de Rosita (#6). Alejandra Casona de Vargas (#3) es _____ de Maria Teresa (#1) y Gustavo (#4). Es _____ de Felipe (2) y _____ de Rosita (#6). Gustavo Vargas Casona (#4) es _____ de Felipe (#2) y Alejandra (#3) y _____ de Maria Teresa (#1). Rosita Vargas Guevara (#6) es _____ de Gustavo (#4) y Julia (#5). Es _____ de Felipe (#2) y Alejandra (#3) y _____ de Maria Teresa (#1).

lets see…so felipe must be the father (el padre) of maria y gustava. he’s the father-in-law of alejandra casona. he’s the grandfather of rosita. im not sure about #4, because alejandra should be the sister-in-law to maria teresa but not to gustavo cause that doesn’t make sense. #5, not sure either. wow i can see where you’re confused. this totally doesn’t make sense. i hope this helped! though i didn’t get very far either…