I think really your issue should be with your brother.
You say he used to be thoughtful about birthdays, and now he's not.
I wouldn't necessarily blame your SIL for that.
DH did this to me when we first married, all of a sudden I was responsible for every card, gift and telephone call to his side of the family.
He is in a job that means he's often away, but it went from "I'm not going to be here, would you mind doing this…" to "why haven't you sent my sister a card?"
We had a big row in the end and now he's responsible for his own side of the family, which means mostly they get nothing from us because he can't be bothered to remember them, and all of them blame me.
Which is ironic because none of them ever remember my birthday.
Blame your brother. Yes she is his wife, and she's your friend, but he's the one who is related to you and who has changed his behaviour and perhaps she thought he had sent you something on time.
It was a bit slack of him to ring two days late, but when he did, and you told him it's not your birthday, perhaps that's why SIL hasn't mentioned it.
He might have put the phone down after speaking to you and said "I forgot youngest's birthday and now she's pissed off" and put SIL off saying anything about your birthday in case she annoyed you by mentioning it.