Witnesses should be over 18, not visually impaired and of sound mind. Ideally, they should not be a family member. However, it will not invalidate the witnessing if a family member is the only person available to witness, provided they are neither party to nor a beneficiary of the documents being witnessed.
The witness must be physically present and able to see the signatory adding their signature to the document on screen. In times when social distancing measures apply, this could mean that the witness observes the signing standing at an appropriate distance or even through a window, provided that it is possible for the witness to see clearly what the signatory is doing. Witnessing remotely by way of video call or similar is not acceptable.