Truthfully speaking, Rebecca knew her niece more than her own parents did since she spend time together with her daughter most of the time.
The outside world thinks the girl is strong enough but she knows the girl is sensitive and fragile deep down. She knows how much Jane loved Edward but the man's betrayal just because of sex hurt her so bad.
She might act ok but she is not at all and this hurt her even if she was just an aunt to her. When she needed help and did not want to rely on her family, Jane came through for them. The entire Lu family never knew what she went through after Jonathan's betrayal so they could not help but Jane did all in her powers, went to a level of establishing an entire company for her daughter's comfort once she hinted about joining the entertainment industry.
When she received news that Edward was heading to country B, she decided to travel too and make things difficult than they already are.
