Pattinson, 26, reconciled with Stewart after she apologised to him following her cheating scandal with married director Rupert Sanders, reported Sun Online.
"The rows kicked off on Christmas eve when protective Lizzy had some words with Robert about getting back with a cheat.
"They were out drinking in Barnes, where Rob grew up. Lizzy was giving him grief about taking Kristen back and said she's always hated the way he's been treated by her," a source said.
Stewart, 22, decided not to accompany Pattinson for his family get-together as she is scared of them.
"He and Kristen spent the festive season apart partly because she's terrified of facing Rob's family," the source said.