It is not mentioned in this scene for what reason why Mahiru's parents can't rely on their parents to take care of Mahiru instead, but it may be for the same reason why they can't get divorced - it would cause trouble if they knew about what they had done. Or maybe her grandparents are scum too, who knows
I've a few friends who ended up in this boat. Parents were disowned or grandparents were abusive and in the end ended up alone with no one to support them. You meet a lot of people with similar stories in foster care.
Some of them made it out okay but a lot fell into self harm, drug use or killed themselves.
On my side, my grandparents on dad's side only found out I was still alive 2 years ago, 30 years after he stopped contact with them. We live continents apart and they're poor so no chance they would've been able to help when I was a homeless teen. Mums side are abusive narcissists, mum especially so.
People in this comment section don't get how realistic this shit is. This is pretty damn normal for me. You should all hug a loved one and be thankful for what you've got in your lives.