A will without 2 witness signatures are invalid in law. However, family members may treat the intentions of the deceased, if the document is bonafide. Registration of will is also not mandatory in all places.
In order to protect the fraud, witnesses are required to sign the will. If you want your will to get registered, you need to visit sub-registar's office along with the witnesses. Witnesses must be two adults and if the will is not signed by them then the judge will decide whether to admit the will or not. A witness must not be the beneficiary under the will, also your lawyer can act as a witness.
Witnesses are usually required because by the time a will takes effect and the person who signed isn't around or not the document that is being presented in the probate court is his will then the witness can and testify in the court that the will belong to the will- maker without any undue influence and with sound mind.