This is The Golden Light of Dawn. It is licensed by Manta.
After losing her family in a demon attack, Rosha is reborn as the 777th most powerful magic knight in the Golden Dawn Society. To avenge her family, she trains hard and rises through the ranks against the odds. Then, she is tasked with protecting little Joshua, who is said to be the Child of God...
There are a number of differences from the question, but the general outline is the same. She loses her family to a demonic attack in the first chapter, to "Ramia", a half-serpent, half-woman demon (not spider).
She escapes with her sister Lily (they're not twins, Rosha is explicitly older, but their birthdays are only off by 1 day) and tells her to run off and stays behind. God grants Rosha power as she's about to die, and kills the Ramia. She's picked up by a man, is told that there's no trace of her sister, and swears she'll find her sister and kill any other demons she can find.
There's no second life involved; she dies and is immediately resurrected by God's power. After a timeskip she becomes ranked 77th instead of 777th - it's possible she reaches #2 later on, but at least at first she's not there. The guy she doesn't like is, at least at this point, ranked third.
The prologue shows the possible child of god, with hints that he may actually be a demonic child instead.