How do we determine Mahram relatives. Is my brother-in-law my Mahram relative?
Mahram relatives are those with whom there exists an eternal prohibition to marry. In other words, they are those relatives with which marriage cannot take place in any circumstances.
The Qur’ān has listed these relationships:
Prohibited to you [for marriage] are: -- your mothers, daughters, sisters; father’s sisters, mother’s sisters; brother’s daughters, sister’s daughters; foster-mothers, foster-sisters; your wives’ mothers; your step-daughters under your guardianship, born of your wives to whom you have gone in, -- no prohibition if you have not gone in; -- [those who have been] wives of your sons proceeding from your loins; and two sisters in wedlock at one and the same time, except for what is past; for God is Oft-forgiving, Most Merciful. Also [prohibited are] women already married, except those whom your right hands possess: thus has God ordained [prohibitions] against you: except for these, all others are lawful, provided you seek [them in marriage] through gifts from your property, -- desiring chastity, not lust. (4:23-24)
This list of course is with regard to Muslim men. With regard to Muslim women, all the male counterparts of the relations outlined in the above quoted verses are understood to be implied.
As far as the second part of your question is concerned, since a woman can marry her brother-in-law if her sister dies, this relationship cannot be classified as a Mahram one.