they didn't
squirrelflight lied.
for one its leafpool not leafstorm and yes they do (lionblaze hollyleaf jayfeather) but everyone thinks that they are sqiurrelflights and brambleclaws kits (even brambleclaw) only squirrelflight leafpool and crowfeather know that though.
Squirrelflight did not have any kits of her own. She raised Leafpool's kits, Jayfeather, Lionblaze, and Hollyleaf, as her own to protect her sister's secret that she had broken the warrior code by having a litter of kits.
Brambleclaw thought the kits were his because Squirrelflight passed them off as his to protect their true parentage, which was revealed later to be Leafpool and Crowfeather's kits. Squirrelflight did this to protect her sister, Leafpool, who had broken the warrior code by falling in love with Crowfeather.
No, Squirrelflight is infertile. She cant have kits due to a past injury; during Eclipse (book 4 in Power of Three) when Thunderclan fought Windclan, she suffered a massive gash to the stomach which, in turn, made it impossible for her to have kits.
No they don't have any kits. You may think that they did have Jaykit, Hollykit, and Lionkit but those are actually Crowfeather and Leafpool's kits. Squirrelflight covered for Leafpool and took the kits.
Their kits where believed to be jaykit/paw/feather hollykit/paw/leaf and lionkit/paw/blaze, but they are really leafpool and crowfeathers kits. Squirrelflight lied and told the clan that they where her kits and not leafpools because the kits were half clan and leafpool was a medicine cat. But hollyleaf died when the tunnels collapsed while she was trying to run away from her clan to start a new life. Squirrelflight tells Ashfur that hollleaf,jayfeather,and lionblaze are not her kits because Ashfur was going to let them burn in a forest fire because he once loved Squirrelflight but she chose Brambleclaw over him so he said to her letting her kits die would hurt her in away.Then Squirrelflight tells Ashfur that they are not her kits and he wont hurt her by killing them. Later you find out that jayfeather and lionblaze are really Leafpools and Crowfeather of Wind Clan kits. Squirrelflight and Leafpool lied to protect Leafpools kits and Squirrelflight told everone that they where Squirrelflights and Brambleclaws kits. In Sunrise Leafpool resigns as medicine cat leaving jayfeather as a full medicine cat and Leafpool as a warrior.
No, Squirrelflight is not infertile. She has had kits in the past and is able to have more kits in the future if she chooses to.
Squirrelflight was foster mother to Jayfeather, Lionblaze, and Hollyleaf, but as revealed in Sunrise, they are actually the kits of Leafpool and Crowfeather. I suppose Squirrelflight is barren because she had been mates with Brambleclaw for so long and still doesn't have kits. But since they are not mates anymore, Squirrelflight probably won't have any kits.
YES. in one of the books, Ashfur tries to kill the Three but Squirrelflight says that they aren't really her kits. Later, the Three unravel the web of mysteries and find out who their true parents are.
Brambleclaw has no kits biologically, since it has been revealed that Squirrelflight did not bear him kits; instead, the kits were her sister Leafpool's. However, his adopted kits (the aforementioned kits) would be Jayfeather, Lionblaze, and Hollyleaf (presumed deceased).
Sandstorm's kits are Squirrelkit (Squirrelflight), and Leafkit (Leafpool).
Squirrelflight is not infertile. She and Bramblestar have kits together in the Warrior Cats series.