I agree with chelle...... as at this time,your brother in law is the only person who is close to his wife. He can talk to her openly and i think if he will talk with her with love and polite manner, she will understand this.Maybe not in first talk, but eventually.If any other or you try to talk with her on this issue,may be she feel u r trying to insult her.
Well, my husband is the good example of this.
When i got married to him, his real sister was the eldest member of his family.As my husband lost his mother at the age of 6. so, for him,she is like a mother.She is 15 years elder than me and i never felt comfort talking with her and she always complained to my hubbby that i did not respect her. Truly, i m not that kind of person but from starting, i felt she is too old and i can not carry big talk with her.At starting we, had lots of fights on his issue but, then i felt my husband gave much respect(well, he gives always)and care about my father and my younger sister. I was so impressed and then i also started to think if i feel so much happy and proud that my hubby is so good that he care about my family then i must give much respect in the way as her sister wanted. Now that issue has become past. Though, my sister in law and me r not good friends but i like her company.One more thing i notice that she gives me much priority to me as compare to his brother(-my hubby) which forces me to never let her down.
Well, i don't know after reading this u will get any idea to sort out this problem but i think every problem has solution and just keep on thinking but don't get disturbed your personal life as you told your husband get upset.
I think celebrating small occasions like your kids b'day or family get together.U may ask her to gives you tips how to manage all that and start doing shopping with her which would make you possible to understand her nature.