What to call women whose husbands are biological brothers?

I would like to know what someone could call the women whose husbands are biological brothers? It isn't sisters-in-law of course, nor wives-in-law!
Can someone help? I am in total confusion.

Answer: co-sisters.

Explanation: In Indian English, two men are called "co-brothers" if they are married to two women who are sisters.

http://dictionary.cambridge.org/dictionary/english/co-brother

So I suppose co-sisters would describe the two women in your case, although this term is specific to Indian English and I am not sure whether it is commonly used even in India:

CAMBRIDGE English dictionary defines co-sister as

noun (...) INDIAN ENGLISH

the wife of your husband's brother

http://dictionary.cambridge.org/dictionary/english/co-sister

EDIT:
I am writing a more detailed answer because I realised these terms are specific to Indian kinship relations and possibly unfamiliar to native speakers of English worldwide:

The term ‘co-brothers’ is more common because it's a special relationship explicitly defined with an English neologism (coined over 100 years ago to refer to men 'newly related by marriage' after marrying two or more women who are sisters by birth) — co-sisters is an exact-parallel term derived from it.

In previous generations, there were large families in India, and today there are families with 5 or 6 sisters whose husbands are all co-brothers of each other — although living apart and technically members of different (their own patrilineal) families, co-brothers were typically closely bound by friendship and obligated to help and support each other.

EXAMPLE:

BOSS: Why you are going on leave? (INDIAN ENGLISH)

Mr.Gopal: I need to go and be with Mr.Krishnan, who is having his appendix removed.

BOSS: How you are related?

Mr.Gopal: I and Mr.Krishnan are co-brothers.

On the other hand, co-sisters traditionally lived together in the same joint-family household of their husbands and 'worked together' under the direction of the mother-in-law. In the Indian context of mostly patriarchal joint-families, no special term was needed for women (now called co-sisters), who technically belonged to the same (marital) family and were grouped together as sisters-in-law in the meaning of daughters-in-law of the same household. So 'co-sisters' is a less commonly used term (more relevant only recently with the increasing prevalence of geographically separated nuclear families) derived from 'co-brothers.' Modern Example:

Mrs.A: I came to know only recently from facebook that you are related to that TV star Mrs.C...

Mrs.B: yes, Mrs.C and I are co-sisters.

Mrs.A: OMG! but why I have never seen you together? (INDIAN ENGLISH)

Mrs.B: That's because she lives in India and we live in the USA!

Note too that there is no special term for this type of relationship (and so the equivalent term is the more generic brothers-in-law // sisters-in-law, as already mentioned in the previous answers and comments) in American and British English.

The wife of your husband's brother would be your sister-in-law:

Sister-in-law

• 2b: the wife of one's spouse's sibling

MWD

Consider the following family tree, where M1 and M2 are male siblings (brothers). They both get married (to two women, very traditional).

        |
     +-----+
     |     |
F1 x M1    M2 x F2

Now:

• F1 calls M2 her brother-in-law and F2 her sister-in-law
• F2 calls M1 her brother-in-law and F1 her sister-in-law
• M1 calls F2 his sister-in-law
• M2 calls F1 his sister-in-law

note: This answer completely ignores the usage of co-brother / co-sister since, as far as I know, that is quite specifically Indian English. The question was tagged to ask about American and British English, notwithstanding the (imho surprising) fact that the answer for Indian English was accepted.