Hasan (may Allah have mercy on him) said: "If someone divorces his wife and she is pregnant, then he can marry her sister after the waiting period ends, and if she is not pregnant, then he can marry immediately."
Hadith Referenceسنن سعید بن منصور / كتاب الطلاق / 2923
Hadith Takhrij«أخرجه سعيد بن منصور فى «سننه» ترقيم الدرالسلفية برقم:، 1745، وابن أبى شيبة فى «مصنفه» برقم: 17022»