Ketubot, Chapter Six, Mishnah Six
This mishnah discusses the dowry given to an orphan girl who was married off by her mother or brothers.
If an orphan was given in marriage by her mother or her brothers with her consent and they gave her a dowry of a hundred, or fifty zuz, she may, when she reaches majority age, legally claim from them the amount that was due to her.
Rabbi Judah says: if the father had given his first daughter in marriage, the second must receive as much as the first.
The Sages say: sometimes a man is poor and becomes rich or rich and becomes poor. Rather the estate should evaluated and [the appropriate amount] given to her.
Usually, only a father has the legal ability to give his daughter in marriage. However, the rabbis gave mothers and brothers the ability to marry off the daughter should the father have died. If the mother or brothers give the daughter a smaller dowry than is typical of a family of their economic status, the daughter may, upon reaching majority age, make a legal claim against her fathers estate and receive a higher dowry. We can see that the dowry is a legal right of the daughters and that right cannot be abrogated by her mother or brothers, who might, after all, have a vested interest in reducing her dowry (since a large dowry cuts their inheritance.
According to Rabbi Judah, if the father had an older daughter whom he married off before dying, the court can force the family to give the same amount to the second daughter. The Sages, however, disagree, for sometimes a family can grow rich and sometimes they grow poor. Furthermore, sometimes a rich father can be cheap and give his daughter a small dowry. While alive, since it is his money, this is his right. Likewise, a poor father might give his first daughter a larger dowry than he could afford. In both cases, the first daughters dowry should not indicate the size of the second daughters. The only way of assessing how much her dowry should be is by correlating it to the size of the estate.