Yes. It’s a sign of co-dependency, controlling behaviour, a lack of respect of other people, paranoia and untrustworthiness, none of which are qualities that people look for in friends and all of which are qualities likely to damage relationships with extended family and also indicate that the marriage itself is highly likely to be an unhealthy relationship. Most people can spot people like this from a mile away so avoid being friends with them.
However, when someone in your family gets into such a dysfunctional relationship it’s more complicated because nobody wants to cut off/ distance themselves from family members they were close previously. However, generally one would have no choice but to distance oneself from them if they behave like this and demonstrate through their behaviour that they think it’s ok to treat their family differently now because they are in a very important relationship and, therefore, in their mind this other person has a right to know everything anybody close to them tells them in confidence. It’s so weird and very dysfunctional. Family members can get sucked against their will into being a pawn in the difficulties these types of co-dependent relationships create and the battles between the spouses over their controlling behaviour and - as evidenced by some posts here - such people seem to think that extended family should accept being treated so poorly just because their relationship with their spouse is dysfunctional.
Sadly the only choice available in such circumstances is usually to distance yourself from that previously close relationship with the family member because people like this seem to think that extended family should treat them and their spouse as one entity and given that they’re highly unlikely to be as close to a spouse as, say, the sibling with whom they’ve grown up and shared a lifetime of experience, if their sibling insists (or their spouse does and the sibling complies) that same boundaries/ level of sharing have to be applied to both of them then the only available option left is to reduce the level of closeness with the sibling to the level of how close they are to the partner and what they’d be happy to share with this person if they did not happen to be in a relationship with their sibling. This is usually not a lot at all unless the spouse has independently established a close relationship and friendship with their BIL/ SIL! I.e. if their marriage split up, would they still see the BIL/ SIL independently and be close friends? If not, then of course they will share information with you on a different basis to how/ what/ when/ in what detail they share things with their own sibling (if the siblings are close), and this is perfectly normal and reasonable. It’s difficult to fathom why anybody would feel entitled to be told everything that’s shared in confidence with their spouse unless it’s something directly to do with their own relationship.
It’s really sad that people do this. I’ve seen it happen so many times. I think perhaps it’s mostly done by people who lack empathy/ have grown up with very dysfunctional models of co-dependent relationships which they emulate, so they presume that because they trust their spouse and share everything about themselves with that person, that automatically means everyone else should be prepared to do the same. It’s quite a bizarre expectation, really. I’ve seen it happen in my extended family several times and it just ends up with the person quite isolated, with few friends and their family relationships becoming far less close/ completely detached. When inevitably many of these dysfunctional relationship break down and people who have behaved like this often find themselves very isolated indeed.
I don’t think the OP’s case is this extreme from what she’s said! I’m referring to some of the weird and quite manic/ disturbing posts on the thread from other posters. I think in the OP’s case maybe she just hadn’t thought through that SIL is not deliberately excluding her specifically and is taking this personally, rather than realising that there is no reason she needed to know about this earlier than most of the family and obviously it’s totally reasonable for her SIL to tell her brother about her pregnancy if she wanted to and not anybody else at that early stage (and there are very good reasons she may have decided to do that which aren’t personal to the OP; she didn’t tell everyone except the OP and exclude her in some way, SIL just decided to confide only in her brother which is totally legitimate if that’s what she wanted to do).
He’s a separate human being, married or not, and this is SIL’s private information that the OP had no need to know at that point in time. Him being married shouldn’t impact in any way on his relationship with his sibling. I wonder how the OP would feel if it was the other way around and her husband told his sister something she told him in confidence, that was nothing to do with his sister?