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.
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.