Needlwork usually involved plain sewing learned at an early age so most women were able to mend and alter clothes for themselves and family members.
Ladies also did fine work by way of embroidery.

Jane Austen was an accomplished needlewoman in plain and ornamental embrodiery. She was accomplished at whitework such as satin stitch.
Whitework embroidery was fashionable in the late C18th, at least in ladies' clothing like muslim scarfs and on white gowns.

Penelope Byrde. 'Jane Austen Fashion' (Excellent Press. 1999).
pp103-108.

Possibly if Charles' shirts were not needed immediately, Jane may have stitiched simple embroidery, perhaps her brother's intials.

In both Letters, Jane Austen also refers to bought clothes-
In letter 19 writes to Cassandara 'I have no objection to your buying our gowns there' (in town) then asks Cass. to buy her some silk in town or in Canturbury.
In Letter 19, she thanks Cass. for marking her Silk Stockings, possibly newly bought.

Clearly, Jane Austen enjoyed cheerfully discussing clothes in her letters. :)