Complex sentence: She insists that Snape take part in the Unbreakable Vow, which she believes he will not except, but when he agrees, this prompts Bellatrix to except his determination to protect the young Draco.
Split into simpler sentences: She insists that Snape take part in the Unbreakable Vow. When he agrees, this prompts Bellatrix to except his determination to protect the young Draco.