What exactly, was the gist of Rachel and Le'ah's response?
Seforno: They were expressing surprise that Ya'akov 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
This is why Yakov called to her first. And even so, one opinion in Bereishis Rabah (74:4) says that she died before Leah for answering first!
What does it mean "do we still have a portion and inheritance in our father's house?!"
Tosfos ha'Shalem (2): Now Lavan has sons, so we will not inherit anything.