because women are naturally territorial. (The same may actually hold true for the sister by the way) Both of them are protective over the man and therefore it is competition at it's worst. Same problem with some mother in laws. However if neither were posing any threat for competition, there wouldn't be a problem.
There is no relationship. You do not share a common ancestor. The connection is simply through marriage.
sister in law
Yes, I suppose that they would be sister-in-laws. Because your sister is your husband's sister-in-law, and the other sister is his sibling, then I suppose they would be sister-in-laws.
Your brother would call your husbands sister by her first name. Or, "my (sibling's) sister in law.
Your Sister-in-law
you call her your daughter's husband's sister
Everyone has a different attitude to these types of situations, I can only answer this personally. I would not go to the funeral of someone I didn't know, simply because I know someone that knew them. The exception to this would be going in order to support the person that knew the deceased (i.e. your husbands sister) if they felt they needed it and you were both close.
Mom
It is simply not true that a majority of the wives and their husbands' sisters don't get along and are mostly at loggerheads. Some women are like that whether they are sister-in-law or not. It has more to do with personalities and culture.
I think that you call them your cousin in law because its pretty much the same concept as a sister in law... its your husbands cousin so it would be a cousin in law
brother in law
There is no relationship. You do not share a common ancestor. The connection is simply through marriage.