What exactly, was the gist of Rachel and Leah's response?
Seforno: They were expressing surprise that Yaakov could for one moment think that it was hard for them to leave their father. 1
As they go on to explain.
Why did Rachel answer first, before her older sister?
Tosfos ha'Shalem (2): She was his primary wife. 1
R. Yosi (in Bereishis Rabah 74:4): It was proper that she answer first, for he called to her first!
Ha'amek Davar: She was in greater danger of dying due to his delay of fulfilling his vow, for she was his primary wife. Perhaps this is why she died first, like it says in Bereishis Rabah 81:1,2! 2
This is why Yaakov called to her first. And even so, R. Yudan (in Bereishis Rabah 74:4) says that she died before Leah for answering first! (PF)
What id Rachel and Leah mean by, "Do we still have Chelek v'Nachalah in our father's house?!"
Tosfos ha'Shalem (2): Now Lavan has sons, so we will not inherit anything.
Ohr ha'Chayim: The two expressions hint that we will not inherit our mother (who already died), nor our father. It is possible that among Nochrim, girls normally inherit with their brothers (Shabbos 116a).
Malbim: Sometimes a man marries off his daughters without a dowry, and after he dies, they inherit a share of his property, like sons. Sometimes a man gives gifts to his daughter in his lifetime, when he divides his property. Lavan did not give to us, and stole what is ours. We have no hope of Chelek (gifts in his lifetime) or Nachalah (inheritance). Hashem already transferred our share to you!
Ha'amek Davar: Chelek always refers to Metaltelim, and Nachalah to land. Surely they did not expect to get land! Rather, esteemed merchandise in a region is considered like land (Kesuvos 67a), in this case, flocks.