In typical Jewish fashion, the answer is as complex and ambiguous as anything - and of course, differs based on how traditional or religious the wedding is. Orthodox ceremonies call for one set of rules while conservative and reform ceremonies call for others.
Traditionally, the Ketubah is signed by a witness to honor, bless and sign off on the matrimony. Generally, the witness is not related to the couple. This rule or guideline applies to all sects of Judaism: orthodox, conservative, reform and even interfaith.
For Orthodox couples, the witness must be a Jewish male of the age of 13 and unrelated to the couple.
For conservative couples, the witness should be male (but are sometimes female), must be over the age of 13 and still unrelated to the couple.
For reform couples, the witness can be any gender, Jewish or non-Jewish but still above the age of 13.
As for interfaith couples, it's essentially a free-for-all. The witness can be any age, gender or religion if the couple chooses to include a Ketubah in the ceremony.
Now that you know who can't sign the Ketubah, it's time to narrow it down to who can! Choosing the witnesses for the couple is usually the hard part but it's important to ask yourselves three important questions:
-Who are each of you closest with?
-Who have each of you known the longest?
-Which of your friends best know you as a couple?
Hopefully these questions help wittle the list down to a select few. Traditionally, both partners select one witness to be part of this beautiful tradition.
Once the witnesses are selected, they join the couple, their families and the rabbi at a small ceremony before the wedding called Tisch (not to be confused with NYU's arts school). There, the rabbi will review the Ketubah to make sure it's valid and review the agreement with both members of the couple. Traditionally, the rabbi will give the groom a handkerchief and have him raise it in front of the two witnesses, a sign of consent and agreement, before handing it back to the rabbi. From there, the rabbi will complete the Kenyan Chalipin, which symbolizes the completion. Finally, the two witnesses are asked to sign the Ketubah and in most cases (with the exception of Orthodox ceremonies), the bride and groom will sign it as well.
After this, the Ketubah is put aside until the ceremony starts, where the Rabbi will read it between the Kiddushin and the Nissan, both important stages of the wedding ceremony. At this point, the couple agrees to the conditions discussed in the Ketubah and it is passed over to a family member who keeps it safe until the wedding is over.
Long story short: The Ketubah is an important tradition that is symbolic and meaningful for many Jewish people and allows the couple to put thought into how they'd like to begin their marriage, much like the vows.