riddle - Who are these two sisters?

Once each month I slept with you,
But unfortunately I'm a young woman.
I'll give it to you square: I'm rich!
Halfway to perfect in an attack.

My sister makes much less sense:
Her misfortune was to be created.
Twice I told her, I really did,

But her perfection has now dimmed.

Who is the helpful speaker?

HINT 1:

We're found in vans, in bags, on bikes - we're found in books, in maps, online

HINT 2:

the answer itself isn't just a letter, but letters (as indicated by the previous hint) are involved in the cluing somehow.

Answer

Answer for the first sister

(with edits thanks to Rand al'Thor hints in the comments):

I can't shake off the sense that each line gives a letter and a word beginning with that letter.
And thanks to the hints in the comments, that the first sister is Aid (hence the helpful speaker!)

Once each month I slept with you,

Once each month = twelve times, that could hint to the 12th letter of the alphabet : L. With "slept with you" this would give Laid.

But unfortunately I'm a young woman.

My initial idea was that unfortunately translates into misfortune/miss Fortune (or Lady Luck!). But following my current idea misfortune may hint the number 13 (for bad luck), corresponding to the letter M. And the word for young woman would be, not Miss, but Maid.

I'll give it to you square: I'm rich!

Or is it: "I'll give it to U square", as in "double U": W. "I'm rich" then is Wealth.
But no! It must end in "aid", so I'll go for Paid, P being the 16th letter (a perfect square).

Halfway to perfect in an attack.

Halfway to "perfect" there is an F. An attack is a Fight.
Or... still going with "aid": a Raid. R because 18 is a semi-perfect number and also half a perfect square (36) as noted by @Statman.

And for the second:

My sister makes much less sense:

All I could find, with a bit of help from @Rubio, is Ade. At first I though "it doesn't make any sense", and then, well... I guess that's the point.

Her misfortune was to be created.

Still keeping the M, created may give Made.

Twice I told her, I really did,

Twice hints for B, that could give Bade (an obscure word I was not aware of, thanks @Rubio)

But her perfection has now dimmed.

It could be Fade because, as @M Oehm pointed, 6 (for F) is a perfect number.

Thank you for all the hints!