What will be your reaction?

Shows that blood is always thicker than water! I have following suggestions:

1. If you have no complaints with her then you just continue to behave normally with your sil.Your sil may have had some issues with your sister which are not known to you. You want to be loyal to your sister, I understand. But don't unnecessarily complicate relationship with your brother by jumping into a misunderstanding between his wife and your sister.

2. Don't give too many gifts to sil. Try to give small gifts which are reasonable but don't go overboard. Your brother is your brother. Your sil doesn't have any rights to tell he will only have contact with you if you give gifts. Just like her parents and sister are enjoying at her house, your sister also should go to their house. Why she is not doing this? Why your brother never invites his sister to his house? It is irresponsible of him to neglect her. When inlaws are there very often, sister can also be there. It is also his house.

3. Maybe your parents can spend more time in India with their middle daughter if she is going through rough time instead of being in US. This way she doesn't feel isolated and that she has no one.Since her inlaws are deceased due to suicide, her hubby may be depressed. So she may be needing a lot of additional support which only parents can provide. Also if your parents are not here, then your hubby will have a chance to bring his mother over especially as she is being mistreated in her second son's house. Just like your parents like to visit you, your MIL also may want to be with her son for some time and enjoy a calm stay in youngest son's house without any beatings and abuse..

 

If you change your attitude based on above expectation then you're just fuelling sibling rivalry & not using your brain... no matter you're related to your sis by blood but ain't you trying to prove blood is thicker than water & hence a reason why a lot of our DH dont support us in front of their parents... no matter who's right or wrong... why a person feels offended/ insulted... should their relative simply isolate the so called OUTSIDER and form a herd of monkeys who may have been fighting internally?

Why should you choose to fight battles for others... this is called isolating and grouping & the very fact that your 2nd sister even expects her siblings to do this shows her maturity levels. A matured sister will simply say.. to each their own ways... dont spoil your relation if mine is not good.