In most cases, the cause of retinal membrane remains unknown. However, it can occur as a result of retinal detachment, trauma, blood vessel abnormaities, inflammatory diseases etc. Treatment is not required if membrane is mild and does not affect the vision like in your case. It's not necessary that you will develop a membrane in the other eye also. You should monitor your vision with the grid provided. If any blurring or vision deterioration takes place then you should report to your doctor. Thanks and best wishes.
I do not think any one will be able to give you the percentage but it is not very likely to develop in the second eye. In most cases it is idiopathic i.e. the cause is not known. There are many people who have this condition in both eyes and many who have it in just one eye. Since, yours is only mild the other eye may not get involved. Thanks.