Fascinating, though not at all uncommon. Let's hope your sister isn't on Medium! Or your brother-in-law! The only part of this that you can meaningfully do anything about is to work on your sibling relationship. I think there are whole periods of our lives when we see our siblings a lot less, if at all, and then there is a coming together (usually prompted by a change in life circumstances, like having children) and you end up closer than ever. Personally I would write to her but I wouldn't make any criticisms of her marriage — just keep it specific to the two of you. Best of luck to you.