Why no option for "other or blank?" Because the program is designed to connect parent-child generations.
The inability to add freestanding individuals can be a minor irritation for those with large trees that are surname-studies, or for trees created for other purposes, but most tree users want to work with family groups.
There is a way to add a person who does not have known family connections. Pick any person in your tree and add the new person as a parent or child. Then go to the new person's overview page, click to "edit person," click "Relationships" tab. You will see listed the person to whom the new person is attached. At far right is a red X that you can click to sever the relationship. Clicking on this will not remove the old person, it will just delete the connection between the two. Now you can add whatever details or relationships you wish to the new person.
Adding siblings where you don't know parents' identities is something that many want to do at one time or another. The MemberTree program just has no way to identify them as siblings without there being some entry for a common parent.
So you have to invent a common parent. Go to your gr gr grandmother's person overview page, click to add one of the parents. You can put in a possible surname and leave everything else blank. You can even put "Unknown" in the surname field or whatever seems logical to you. As soon as you save this place-holder you have established a parent, and the program will allow you to add a sibling of your gr gr grandmother. As you learn more, you may be able to fill in information about the parent. Don't worry about having something "wrong" in your tree here -- it is all a "work in progress" and easy to edit when research results allow adding data.