Act 2 is all original lore that Larian wrote for what happened between Bg2/Bg3 for the area!
As for Aylin herself -
While Aylin describes herself as an aasimar, she's a very specific type of aasimar, which is confusing if you only know of aasimars as the celestial equivalent of tieflings. By her own description, she would be a deva: she's an immortal, angelic soul that is able to be "reborn" again and again after she dies. There is several different types of aasimar/devas and it gets complicated since there is several different varieties, but the names are often used interchangeably.
There are:
mortal aasimar (planetouched origins, the equivalent of tieflings, usually of human ancestry)
Aasimari Devas (immortal angelic souls in a mortal humanoid form)
and "angelic devas" - celestial angels and servants of the Gods, usually separated into three different orders.
I am not sure if current lore really differentiates Aasimari devas and angelic devas anymore, though - the difference between them is... somewhat minimal and it gets confusing when they have virtually identical names.
If you ask Aylin if she is literally the daughter of a god, she says that she was "ripped from Selune's celestial womb" - quoted roughly from memory - and considering devas can be the servants of Gods, it makes sense that the Gods may create devas themselves. Usually aasimari devas are suppose to lose their memories when they reincarnate and they don't always have the same appearance, but Aylin might be a special case as narratively it wouldn't work quite as well if she didn't even remember who she was, or if Isobel hadn't recognized her. Fun fact: Aylin is a Turkish girl's name, and it means "of the moon."
For Act 2, a lot of Ketheric's background is buried in journals and letters:
Act 2 spoilers: Ketheric, his wife Melodia, and Isobel were all devoted Selunites. Isobel meets Aylin and they fall in love, and Ketheric was disapproving of the relationship: Aylin is an immortal by virtue of being an angel, and there is the added unequal power dynamic that she is the child of their god. Something happens which causes Melodia to die; Ketheric stays strong for Isobel. Isobel later mysteriously dies; Ketheric is consumed by grief and despair, and as he is unable to resurrect his daughter, he forsakes Selune and is claimed by Shar.
This appears to be roughly when Ketheric fools Aylin and binds her in the Shadowfell, to be used for the ritualistic ascension of Shar's Dark Judiciars: they are able to kill not just an ordinary Selunite, but one of Selune's prized daughters/servants again and again. Presumably, Aylin may have been involved in Isobel's death in some way (thus getting Ketheric's likely unjustified ire?), but we get no confirmation on this. Ketheric's sights on Aylin may have been due to his fall to Shar alone; Ketheric mentions that Shar was unable to take away his pain, so he may have gone to far greater extreme's to get a reprieve that never came.
The battle between the Sharrans and the Druid's Grove, Jaheira, etc. take place sometime after and Ketheric died. At some unknown point, Ketheric is claimed by Myrkul and is resurrected. Myrkul also brings back Isobel, something that Ketheric was prevented from doing previously. I forget who said this, maybe it was Jaheira, but they comment on how he changes his allegiances quite a bit.
You can tell Ketheric that Melodia would be disappointed to see everything he's done, and that it "isn't too late." Ketheric disagrees, and depending on the dialogue choices, may willingly fall into the pit to his first death without combat.
Act 2 is primarily content for Shadowheart, honestly, but the rest of her storyline plays out in Act 3. Honestly I suspected that
she was going to be Isobel's sister or something, especially since they're both high half-elves and one belonging to Selune and one to Shar would have been interesting
but that wasn't how it ended up playing out.
I thought the reunion between Aylin and Isobel was cute.
As for the resurrection itself:
There is many different rules and limits when it comes to resurrection magic. Ketheric was a paladin, so the extent of his magic is going to be lower than a cleric, but spells like raise dead (one that he can get) require:
1) person to not be dead for more than 10 days,
2) doesn't remove magical diseases and curses,
3) closes all mortal wounds but does not restore missing body parts; if it is missing something integral for survival, the spell fails.
Depending on the circumstances of her death, a spell like wish or true resurrection may have been the only ways to bring her back. Resurrection (7th level) may have been able to bring her back if it wasn't a magical curse/disease of some kind, but it is not a spell he would be able to cast himself, and finding people able to cast higher levels of magic would be quite difficult.
