What legal and medical documents do I need to get in case something happens?

If your grandmother is fully aware and in her right state of mind, try to get durable POA which gives you lots of rights even when she becomes incapacitated medically and/or mentally. If it isn't a durable POA, your rights end when the person becomes incapacitated. Also, health proxy allows her to appoint someone to make decisions about her health care if and when she becomes incapacitated. The health care proxy has the right verbally and written consent for DNR, to withhold or not IV's and feeding tube, etc. These are your basic papers. If you do not have them, first they may look to see advice from her adult children vs grandchild, but every doctor/person is different. If none of that is obtained, you will be between a rock and a hard place and will have to go for guardianship which is expensive lengthy court process and the person has to be deemed incompetent and have neuro psychiatrist evaluations. Just remember a POA and health proxy can only be done if there person it competent to understand what they are signing. Once, POA and health proxy are obtained, make sure they are sent over to any financial and medical institutions because trust me as I'm going through it, it is a hassle when you need things done quickly, better to have it set up.  I hope this helps.