What did Lavan mean "rather, than give her to someone else"?
Targum Yonasan: He was tricking Ya'akov by throwing him off-guard (in his plan to give him Le'ah), by implying that there was another suitor who wanted Rachel, but that he preferred him. 1
Tosfos ha'Shalem (2): It is better to give her to you, even though you do not give to me anything, than to give her to someone else, who would give to me much money for her.
See Or ha'Chayim. It is also possible that the trick Lavan was playing on Ya'akov was to get him to remain an extra seven years (by giving him Le'ah first), so that the well, which began flowing the moment Ya'akov rolled the stone off it (refer to 29:10:4:1), would continue to flow for another seven years - See Targum Yonasan on Pasuk 22). (EC)
Why did Lavan, an idolater, agree to give his sister and daughters to Yitzchak and Yakov? He knew that they will not let them serve idolatry!
Tosfos ha'Shalem (1, citing Imrei No'am): R. Yehudah ha'Chasid asked his father, who answered that the idolaters believed in Hashem, just they served idolatry to know future events.