There are a lot of people here saying, "Just because she married your brother, she doesn't have to be your friend" and "Why would her older kids care about your younger kid?"
These are supportable views but they're not universal.
I think that when you marry someone, you are connecting yourself to a web of relationships that have been there for many years before you came along, and which are stronger and more meaningful than your new one. You hope that your marriage will grow to be as strong and as permanent, but at the beginning you're the weak link. So it's a good idea to build connections with the other members of the family.
But it's not just strategically a good idea. It's socially a good idea, and you'd hope it'd be fun, and easy, and natural. Because these are the people that made you partner who they are.
Also - and I know this isn't true of everyone - I wouldn't (and didn't) marry anyone who didn't get on with my family and want to be a part of it, because my family is the biggest part of my life.
So maybe that's the measure. I think one has to make an effort to be as close to one's partner's family as the partner is.
To get back to the OP - do you think your brother has become more distant under your sister-in-law's influence? Or is she just adopting his attitude towards you?