Does the sister deserve a share of her mentally ill elder brother's inheritance, in return for her care and spending on him for seven years, and is the consent of some heirs sufficient for this, or must the objection of one of them be taken into account?

If your brother is not good at managing his money, he is legally interdicted. His case should be brought before the Sharia court to appoint a guardian for him if he has no living guardian or appointed trustee. Nothing from his money should be given to his sister merely based on her claim. If she has a financial right, she must present it to the court for proof and assessment. The default is to prevent taking from his money except for his own maintenance.