The brothers say, If she is a wall, we will build on her a battlement of silver, but if she is a door, we will enclose her with boards of cedar.
The point is that this girl, when she gets to the age where she is interacting with boys, will show that she is not a commodity. It is important that young people learn to be respectful of the feelings of love and sexuality. It is very important that they learn to see that the full experience of sexuality is given by the Lord and only has its place within marriage.
The brothers will build a silver stronghold on her as an act of honour. They will honour their sister if she keeps her virginity until marriage. This is the will of God. The brothers let it be known that they will react differently if their sister does not live this way, showing to be sexually available to boys before she is married. Then they will: enclose her with boards of cedar.
You can also translate the word enclose
with to constrain. If their sister is going down wrong paths, it will be necessary not only to talk, but also to take measures that will ensure that she does not easily come into contact with those boys again. Clearly, parental authority may also include prohibition (for a certain length of time) to associate with certain boys and curtail certain freedoms if they are abused. They will choose this to protect the child from her own loose sexual feelings. That is something quite different from what is emphasized in society in our day and age, namely that one should always be able to give in to their own feelings.
9 If she is a wall, we will build on her a battlement of silver, but if she is a door, we will enclose her with boards of cedar.