Just to bring about drama, creatives have made illogical changes in sumitra and it looks odd. One cant hate sumitra as she is one of the most innocent characters of BV. Remember tipri episode?
I am taking sumitra's sudden turn with a pinch of salt.
Sumitra's objection should be only ratan's presence. Thats a logical reasoning. She should have given that card only than the business of getting angry with ganga and especially with mannu as surabhi says.. She could have had a cool talk with ganga, make alternate arrangements for her if necessary. Even then Jagya would have stood between sumitra and ganga. The problem for sumitra is not ganga but Jagya. By now she must have realised that. Even during gauri phase, more than gauri, its Jagya and his love for gauri was the reason for him to move away from his family.