Without a paper trail of any kind detailing that it was gifted to your mother and then to you, you're probably not on very solid ground to be real. The better standing is as a resident/tenant (whether you actually pay any rent or not) you are still a tenant on the land your grandmother owns, and staying there with her consent. You can't be summarily tossed out without an eviction process.
The larger problem is your extended situation with your sibling relationships. This will require real effort and communication to resolve and though you've stated that it's hard to talk to them, why don't you consider writing them a personal letter explaining your feelings about the whole situation? You express yourself here beautifully. Pray and ask God for the wisdom and words to best convey your feelings and seek reconciliation and healing in your family. This might be the quickest, easiest path requiring a humble acknowledgment of your circumstances and personal helplessness. God is mighty-- ask Him for help.
"...Where does my help come from? My help comes from the Lord, the maker of heaven and earth." Ps 121
Lastly- if you truly don't feel safe in the environment, you NEED to change it. Ask to speak to a social services counselor (social worker). This is what they do. If you convey that it is a safety concern they are required to get involved. They cannot and will not ignore it.
Another way to change your environment is to ask for the strength and courage to step out on your own. As I mentioned you communicate quite well electronically-- there are many work from home job opportunities available in areas such as customer service, online support services, order entry, and a myriad of computer-based avenues that might provide an avenue for you to escape the limitations of your SSI benefits as your primary source of financial stability. You said you have friends elsewhere that love and support you, maybe it's time to move away from the circumstances that have you feeling trapped and vulnerable.
May God grant you wisdom in dealing with such difficulties.