The age of witnesses to NSW marriages in that era .... that were up to the clergyman to determine ... it was more important for the clergyman o know that the witness understood their role .... I have quite a collection of CofE marriages for rural NSW 1856-1895. I am sure that some of the witnesses were as young as 12 or 13....

particularly if family members. Even in the 1960s, in NSW I attended a cousins wedding ... and her younger sister, born 1957, was one of the two witness signatures on the NSW BDM registration.

I think the Martha born 1869 would be too young to e a witness at Mary's wedding ?
ADD, one of my ancient rellies, retired NSW bdm senior officer has phoned as I posted. Yes, the NSWBDM info received from the clergy registering a marriage did not require the age of the witnesses. Up to the clergy or celebrant to determine their competence... e.g. even today a very drunk best man would sign as witness, in front of the invited guests but a third person may be discreetly asked to counter-sign, avoiding embarrassing the groom's choice. But if bride or groom are "too drunk" then the ceremony should not proceed. The age of witnesses has not really been tested under the no fault divorce laws, but it was tested in NSW Supreme Court in 1800s. Chief Justice Stephens ruled. My rellie will check and if he can find the ruling, he will let me know. I will post when I hear back.
JM