If that person is capable of posting this question, he is capable of meditation. If he is capable of doing meditation, he can work on attaining freedom from suffering. Taking care of such a person is good Karma for his or her brothers and sisters.
If the patient is not mentally disabled, he should try to cultivate good qualities to make it easier for his brothers and sisters to look after him.
- Always talk politely
- If you want something, request kindly and not demand
- Limit the requests to what you actually need and not become too much of a burden by going after unnecessary desires
- If possible, find work that you can do online to contribute to the family income. Obviously this is very much dependent on how disabled the person is.
- Do good deeds with the help of others whenever possible and wish for freedom from suffering
Suicide is not advisable, but a case can be made for rejecting medicine if there is no chance of recovery and one is totally disabled. Because not taking medicine is simply deciding to live however long the body would survive naturally and not doing anything to extend it artificially. So it's not 'suicide' per se. Then again, there was Stephen Hawkins who became one of the greatest scientists while even not being able to speak on his own. So the course of action is subjective and a blanket answer cannot be given to fit all scenarios.
As for the brothers and sisters, they should show patience, kindness and compassion towards the patient and try to get him involved in good deeds like meditation, listening to the Dhamma, discussing the Dhamma, donations etc.