One has to be careful inferring too much from small words.
“The” and “a” are such words.
If the marriage announcement indicated that the bride was attended by “a sister,” does that mean she had more than one sister. If a the reference to the groom indicated that “the brother of the groom was his best man” does that mean that the groom only had one brother?
One would have to assume that whoever wrote the item knew the family well enough to know who had more than one sibling and who did not. One would also have to assume that there were no errors in the account of the wedding published in the newspaper.
Wedding announcements are not the only references where “a” and “the” may suggest how many siblings (or other types of relatives) a person had. Just keep in mind that while “a” usually indicates there are others and that “the” indicates there is just one, that errors can easily be made.