No, it isn't allowable for brothers and sisters to marry. It may have been possible at the time of Adam and Eve, to get the world's population started. But it has been forbidden since Moses' time, according to Leviticus 18:9.
Your sister's husband is your brother-in-law. Your brother-in-law's brother is not related to you and neither are his children. But he is your sister's brother-in-law and his children are your sister's nieces and nephews.
Your sister's sister-in-law is either the wife of her brother or the sister of her husband.If she is the sister of your sister's husband, then her children are not related to your children.If she is the wife of your sister's brother (yourbrother), she is also your sister-in-law and her children are your children's first cousins.
Your dad's brother is your mom's brother-in-law. He is not related to your mom's sister or her children.
If your brother (or your sister) has children they are your nephews (males) and nieces (females).
A brother and a sister, both being children of the same parents, are siblings.
The children are both your cousins[uncle's children]and your nephew and niece[sister's children].
No here they are still brother and sister, but they do have a stepfather.
Your mother's sister's children are your 1st cousins.
Insest
No, Kris Letang doesn't have any children.
He would be your brother-in-law. To your children, he would be their Uncle.
Your sister-in-law is either the wife of your brother or the sister of your spouse. The children of your brother's wife are your nieces and nephews, since they are also the children of your brother. In some cases they may be your sister-in-law's children from a previous marriage. Then they are not related to you at all. However, if they are very young, it may be convenient to call them your nieces and nephews anyway. The children of your spouse's sister are always your nieces and nephews.