Maybe the legal adult age was 21 back then? This is a Guardianship (trusteeship nowadays) of an Estate- Not an adoption. Guardianship of an Orphans estate is appointed for minors, by the court, in compliance with the deceased's will. That is strange wording saying young orphans chose the guardian. But perhaps the will gave choices as to who could be designated. Cant say without reading the actual document. The Court would ensure this was observed as part of the probate process. Guardianship of an Orphans Estate involves the administering of the orphans inheritance, not becoming their adoptive parents. The bond that the guardian (and the two others who helped him raise the money for the bond) entered into, is the courts way of ensuring that even if the estate was destitute the bond amount would cover the maintenance of the orphans. So the guardian and two others guaranteed the amount of $10,000 . If the estate didn't have sufficient funds to meet the obligation, the guardian and the other two named in the bond would be on the hook for the funds.
The second part (a month later) is dealing with the younger brothers, appointing a Guardian and again this Guardian of the Estate of an Orphan and two people who backed him with funds to meet the $3000 obligation, entered into the bond with him. Why this is dealt with a month later is not clear from your info.
Hope that helps.