I'd say you might need to ask an antique-clothing forum more than a camera forum, but here's my ponderings.
The one on the left is in all black with a bonnet. Looks to me like a 'mourning dress', which (depending on ethnic-origin / religion etc) was probably worn permanently after her husband had died (though not always, my gf's grandma with bavarian origins got married in a black dress like that).
The one on the right also looks like you'll have more luck with dating the dress. Although i think I see a signature on it? How much do you know about photographers who might have taken it, chase it that way?