This is where we learn about all the members of Mara's immediate family. If you want to know about Luke, you can find the information on him in the Luke section.
Ben Skywalker - the son
Ben and Mara have the kind of relationship that you would expect of any normal mother and son. In some ways, she's much more protective of him than would be usual because of that whole nearly-died-giving-birth-to-him thing. In Legacy of the Force, we see that she's somewhat given up trying to protect him so much because she realizes that his destiny is to be a Jedi and there's only so much that she can hide him away from now that he's growing up into a young man.
With Luke as the 'fun' parent, Mara seems to take the role of the strict mother, punishing Ben when he steps out of line and making sure he doesn't eat too much gelmeat. All Mara really cares about is that Ben is safe, and with her last breath, she spoke of protecting Ben from the kind of world Jacen wanted to create.
Leia Organa Solo - the sister-in-law
Besides being Mara's sister-in-law, Leia also serves the role of best friend. With all the hanging around smugglers and the scum of the galaxy, most of her friends up until marrying Luke had been men. With her entrance into the Skywalker family (despite that whole silly having sworn to kill Leia's brother thing), she finally gained her first real female friend.
Throughout the series, we see Mara and Leia's friendship and trust for each other grow (though I'm sure babysitting Ben was an iffy topic around the Skywalker household after that whole kidnapping incident). Even with the Legacy series putting them on opposite sides of a war, Leia and Mara remained close.
Han Solo - the brother-in-law
It would seem as if Luke and Leia have the same choice when it comes to what they like in a spouse--if anyone in the Solo family is like to Mara, it's Han. They were both smugglers, neither is extremely skilled at containing their anger or frustration, both like carrying around blasters more than lightsabers, and they've both got bad pasts. But hey, someone's gotta do the job of counteracting that Skywalker sedateness, and Han and Mara do a damn good job of it.
The two of them didn't see a whole ton of interaction, but from what we did see of them together, they seemed to get along well (which can be attributed to all the similarities listed above).
Jacen Solo - the nephew
Mara's oldest nephew, Jacen, sees just as little interaction with her as Han does, but we do know that she trusts him deeply--there had to be a whole lot of it there for a woman so protective to put him in charge of her only son. When Luke tells Mara that he allowed Jacen to take Ben along with him on the mission to destroy Centerpoint Station, she reacts much less harshly than Luke would have expected from her, and it's no mystery that that had a lot more than little to do with the fact that she knows Jacen can take care of Ben.
But Jacen's turn to the path of the Sith in the Legacy series brought about the eventual erosion of Mara's trust in her nephew, and rightly so--the bitch kills her! Demented nephew, for the LOSE!
Jaina Solo - the niece
Of all of the Solo kids, Mara is the closest to Jaina, and the reason is pretty clear. At the beginning of the New Jedi Order, we learn that Jaina's been put under Mara in a Jedi apprenticeship. The two appear to be very close to one another and Mara becomes a big sister sort of figure to Jaina, who at the time doesn't really have an understanding with her mother.
Even though
Jaina leaves her apprenticeship early on to become a fighter pilot in Rogue Squadron and later take an apprenticeship under Kyp Durron, their strong relationship lasts throughout all the books post-NJO as well. Their bond, both as master and student and as aunt and niece, was there until the last, and Jaina regrets that the last thing she and Mara talked about was something fickle as her relationship troubles.
Anakin Solo - the DEAD nephew
After Jaina left her, Mara more or less took on her younger brother, Anakin, as a student in her stead. But it went both ways, because while Mara was teaching the youngest Solo about not relying on the Force, Anakin was... well, saving her ass from hoardes of angry Yuuzhan Vong. If it weren't for Mara, Anakin wouldn't have been as strong a character as he was when he died and if it weren't for Anakin, Mara would be dead.
As they camped out on Dantooine, they developed a strong bond with one another that lasted up until the time he died on Myrkr. When Anakin met his end, Mara was hurt, but she decided to try to contain her feelings on the subject so that she could be the support that Luke needed.