Elderly woman in charge of a girl on social occasions

One word substitute is Chaperon.

Spinster: an unmarried woman, typically an older woman beyond the usual age for marriage.
Matron: a woman in charge of domestic and medical arrangements at a boarding school or other institution.
Chaperon: an older woman responsible for the decorous behaviour of a young unmarried girl at social occasions.
Chandler: a dealer in supplies and equipment for ships and boats.