While doing research for my writing, I found some of the family relationships to be a bit confusing, so I wrote it out for myself, including my theories and reasoning for some relationships, just to keep everything straight and have it in one place.
Parkers

Liz Parker |

mother Nancy Parker
| 
father Jeff Parker
|
never mentioned |
never mentioned
| 
grandmother Claudia Parker - deceased
|
grandfather unknown - deceased? |
| 
great grandfather Pete Parker - deceased? |
It is unknown if Claudia Parker was Pete Parker's daughter or if she married a son of Pete Parker. Parker could be her maiden name and she never married, but most believe that she married a son of Pete Parker, taking the Parker name.
Liz also has an aunt, who lives in Florida, name unknown. It is never said in the show if she is Jeff or Nancy's sister, but most people accept that she is Nancy's sister, because she didn't come to Roswell when Claudia Parker was dying. Since no other people came to see Claudia, most believe that Jeff is an only child.
Evans

Max & Isabel Evans |

adoptive father Philip Evans
| 
adoptive mother Diane Evans
|
grandfather - unknown grandmother - unknown |
Isabel mentions that they have a grandmother and grandfather, but it is unknown if they are their mother's or father's parents. Since only one grandmother and grandfather is mentioned, perhaps the others are deceased.
Max mentions in 'Leaving Normal' a cousin. It is unclear if he really has a cousin living in Roswell, or if it was a fabrication. But if he does have a cousin, then either Philip or Diane have a sibling or siblings.
Delucas

Maria Deluca |

mother Amy Deluca
|
father unknown
|
grandmother - unknown
grandfather? - Breepa Deluca deceased |
|
|

cousin Sean Deluca |
mother unknown
|
father unknown
|
grandmother - unknown
grandfather? - Breepa Deluca deceased | |
It is unclear if Deluca is Amy's maiden name, or if she married into it. Maria's father is mentioned, but his have was never given.
It seems strange to me that Maria's father ran out on them, and they still keep in touch with his side of the family, (if the Delucas are his family) but haven't seen or heard from him since Maria was six. He doesn't contact any of his family? That is why I lean toward the theory that Amy's maiden name is Deluca.
In an early version of a script for 'Monsters', Amy says her name is Miss Deluca and she is a single mother, so probably she went back to her maiden name and the Delucas are her family.
Sean is Maria's cousin, but it is unclear if he is from Maria's mother's or father's side of the family, and his parents are never really mentioned.
We know Maria has a deceased relative named Breepa Deluca, but it is never mentioned how he was related to her. But since Hal in 'Summer of '47' reminds her of him, I think it is safe to say he was her grandfather.
We also know from 'Wipeout' that Maria has a living grandmother, but it is not mentioned from which side of the family.
We discover in ARCC that Amy was seeing Curt Pressman before she was 18, and we know she was married at 17. Amy also mentions that she did crazy, stupid things with Maria's father when she was Maria's age (DB), so possibly he is Maria's father.
Valentis

Kyle Valenti |

father James Valenti Jr.
|
mother Michele Valenti
|

grandfather James Valenti Sr.
|
grandmother - unknown deceased
| never mentioned |
never mentioned |
Duprees

Laurie Dupree |
mother - unknown deceased
|
father - unknown deceased
| 
aunt Meredith Dupree
| 
uncle Bobby Dupree
|
never mentioned
| never mentioned
|

grandfather Charles Dupree deceased
|
grandmother Ada Jane Dupree deceased |
|
Laurie's parents are dead, but they are never named in the show. Her Uncle and Aunt, who are brother and sister are her father's siblings. Laurie's grandparents, and Meredith and Bobby's parents are Charles and Ada Jane Dupree, both deceased.
Crawfords/Whitakers

Nicholas Crawford Vanessa Crawford Whitaker (adopted) |

mother Ida Crawford
| 
father Walt Crawford
|

Vanessa Crawford Whitaker
|
husband John Whitaker deceased
|
|
step-son (no name given) |
Of course we know that the Skins weren't really related to each other, but for their cover story they were living in 'families'. Here is the fake relationships of the Crawford family, and Vanessa Crawford's real marriage.
Vanessa married a Congressman with the intent of killing to take over his political position. At one point, Courtney mentions that Vanessa has a step-son, who must be the son of her husband. No name is given on the step-son, and we don't know what happened to his mother.