Dating my ex sister in law
If it is something that just takes place later in years there is nothing biblically wrong or unlawful about it, although some may feel it is politically incorrect and may cause distress among family members.
The people they marry are your brother-in-law or sister-in-law.
There is nothing biblically nor unlawful about marrying your ex sister in law. If your brother died and his wife was barren the younger brother would take her as his wife.
Your brother's wife- Again since someone cannot be married to two people at the same time you must be asking if you can marry your brother's ex-wife which is your "ex"sister in law. There is nothing Biblically wrong nor is it unlawful to do in the unfortunate event of the death of your brother.
As long as you aren't still married to your wife, it would be legal. You can marry her provided she is not still married to your brother.
If your sister-in-law is your wife's sister, her sister's daughter could be your daughter, whom you may not marry.