I mean, the obvious surface answer is family, obviously, but it's more specific than that. It's not just a vague 'protect my bloodline' thing. Her entire world, her social standing, her literal survival, is tied to Lucius's position within Voldemort's ranks and Draco being the perfect pure-blood heir. When that starts to crumble—Lucius failing, Voldemort moving into her home, giving Draco an impossible task—her motivation sharpens into a desperate, purely survivalist instinct. It's not about the ideology anymore, if it ever truly was for her. The moment in the forest where she lies to Voldemort about Harry being dead isn't bravery for a greater good; it's a calculated gamble to get back to her son faster. She uses the chaos Harry created to save her own skin and Draco's. That's her real arc: from a complacent, prejudiced aristocrat to a cornered animal doing whatever it takes, even betraying the Dark Lord, for the one piece of her world she has left.
People talk about Snape's love, but hers is just as narratively important because it's so stripped of romance. It's brutal, practical, and completely self-interested in the best way. She doesn't have a change of heart about Muggle-borns; she just stops caring about anything except getting Draco out alive. That feels more real to me than a sudden redemption.