Boba Fett, according to Star Wars Adventures #4-6, a Star Wars Gamer article, and the Star Wars Tales #7 comic, is said to be married to Sintas with a daughter named Ailyn Vel, who wears the Boba Fett armor during the Young Jedi Knight books.
"Never so much held a woman in his arms since Sintas."
"She knows as much about the man as any wife knows about her husband, for in addition to being comrads-in-arms for a while long ago, they were wed and had a child. As with all things surrounding Fett, this is a closely guarded secret, and he will go to any length to keep it that way."
Source: Star Wars Gamer #6 magazine, p. 72
"Boba Fett recovers a hologram disc at the end of a long chase. Upon viewing it, he discovers a message from a criminal named Pizztov who is wanting to sell Fett the true item of his search, a hologram box. It would cost Fett 6.3 million credits and leaving Pizztov alone forever to get it back. He is told that the item will be sold to the Auction Master of Eport if he doesn't purchase it soon. Fett heads to Bidamount to head off the package, even as another person hunting it arrives. The new player is Sintas, a woman armed for a one-woman assault on Pizztov. She bursts into Pizztov's suite and kills everyone but Pizztov and his tramp. One of the goons thought dead rears up to kill her, but is killed as Fett arrives and shoots him. Now both Sintas and Fett hold Pizztov at gunpoint, while Pizztov uses the girl as a shield. Pizztov tries to make some sort of deal, but Fett wants the hologram to destroy it, while Sintas wants it because she says it is her property. Pizztov makes a run for it, and Sintas shoots him. She recovers the box, but Fett grazes her arm with a blaster shot to make her drop it. Pizztov takes Sintas' blaster and fires on Fett, but his blaster skills are as low-level as his intelligence for going up against Fett in the first place. Fett takes him out in one shot. Fett uses Pizztov's retinal pattern to open the canister, revealing the hologram. We finally discover why it was so important to both of them. The hologram is of Fett with his arm around Sintas . . . who is holding, to all appearances, their child! (That's right. At one point, Boba Fett and Sintas were married and had a child.) Fett tosses the emitter to her, simply telling her not to lose it again. He then takes Pizztov as he leaves. Sintas: You're just gonna leave me here . . . wounded? Fett: Yeah . . . your wounds will heal. Sintas: Some never do."
Source: Nathan Butler, GalacticSenate Forum
" Of course what really happened was that Boba left bounty hunting after ROTS and became a Journeyman Protector on Concord Dawn, taking the name of Jaster Mareel. Here he presumably partners with the droid BL-17 and married Sintas, a fellow hunter-turned-upstanding citizen. After killing his superior officer, he returns to hunting with his droid, posing as a stormtrooper for his first assignment."
Source: TalonCard, TFN Forum
"We don't know that he actually married Sintas."
Source: The2ndQuest, TFN Forum
"Actually 2ndQuest, the Gamer article Usual Suspects states clearly that Fett and Sintas were married."
Source: Abel, TFN Forum
"Boba seems to me to be a cold man, and his split with Sintas isn't unexpected. He married her, accord to the CUSWE, and then split. I wouldn't expect him to marry a woman and have her bear his child because he lvoes her, but rather, with his cold personlity, he did it only because he believed it wa necessary for him to pass on his legacy to a son as Jango had done. His philosophies would seem to match this, focusing on what is necessary rather than what he desires, such as love. He probably chose SIntas to be the mother, however, because he does truast her and admire her, but he knows it can't help him to cling to her. I think he will go back for his child once he/she is a teenager or so, to begin to introduce them to Mandalorian techniques, Sintas alreading taking care of bounty hunter training. This would sound just like Fett to me."
Source: Corran Horn, SW.com Forum
"Apparently, every time you see Boba Fett in the YJK series, it's actually Sintas."
Source: jSarek, SW.com Forum
"No, you mean Ailyn Vel, not Sintas."
Source: Darth NTM, SW.com Forum