Biggs, Wedge, and Jesse were the three main members of AVALANCHE, working under Barret to blow up various Mako reactors. The three of them were killed while trying to keep the pillar supporting Sector 7 from falling.
Marlene is Barret's adopted daugher; her biological father was Dyne, Barret's best friend. Barret thought that Dyne had died when their town was attacked by Shinra, but he actually just went a bit batshit. For more about Dyne and Barret, check the Main Characters page.
Marlene seemed rather shy in the original game, and rather more willful in Advent Children. She spent the majority of FF7 with Aeris's mom, Elmyra, which may or may not explain the fact that she looks exactly like Aeris in AC. Also in AC, she got to hide under Vincent's cape, which was just bloody adorable.
Elmyra is Aeris's adoptive mother. She found Aeris on the train platform in Sector 7, crying over her dying mother. Lonely with her husband away at war, Elmyra decided to take Aeris in. (See the Main Characters page for more info.)
After Aeris was kidnapped by Shinra, Elmyra took care of Marlene at Barret's request. At some point or other, Shinra captured the two of them as well in order to be used as ransom to keep Cloud and co. cooperative. That was mentioned once, briefly, and then forgotten about.