Why is there an extra 'Shin' in Yisachar?
Rashbam: It denotes a double reward, one for giving Rachel the Duda'im 1 , the other, for offering her maidservant to Yakov.
Why does Leah attribute her son to giving her Shifchah? Rachel did so, and did not have a son [yet]!
Riva: Rachel did so for she had no sons. Leah already had sons.
Why do we pronounce only one Sin in Yisachar?
Riva, Da'as Zekenim, Hadar Zekenim #1: One Sin is because she bought the right to lie with Yakov, and this is dishonorable. 1