I have my bfs passwords and I wouldn't reply to any emails from his family members and he wouldn't to mine either. But if your brother is okay with his wife doing so, it shouldn't be much of an issue, I wouldn't think anyway.

separate accounts, we don't know each others passwords. but sometimes it keeps us logged in and we accidentally reply to the other persons email. yesterday hubby replied to an email his mom sent me, not realizing- and now she's confused even though i explained to her it was him not me. ugh.

to me email is private. what if you can emailed him to tell him something your mom said about his wife or something? i mean, stuff like that does happen. i never thought to be careful that what i was sending would be read by someone else i guess.

We have separate accounts but the same password. I don't care if DH responds to an email that I get that is time sensitive and I'm not around (as in, do both of you want to come over for dinner on the 10th?) OR if it is from mutual friends. We identfiy ourselves, tho, and say something like "Kimmy forwarded this to me, and we won't be able to make dinner on the 10th".
I don't care if DH opens my mail and this extends to email, and vice versa. IF I am expecting say a receipt from an online purchase for a birthday gift, I'll ask him not to check my emails for a few days.
Personally, it make no difference to me. I can't tell you how many programs I've seen where one partner has "secret emails" from affairs or others. I have only one account and DH has access to it. The same goes for DH, and I have to trust that this is true. And we tend not to pass family issues on to others via email simply because it is possible for emails to be misdirected or shared in ways we never originally intended.

I must be a major exception.We both have multiple email accounts, cell phones, pagers and such. I have no idea what his passwords are and he does not know mine.In fact we have security settings set on all devices.To me this is all very personal.He is a CFO of a large company and I have patient information on all of my devices.I take confidentality very seriously so maybe we are an exception for this reason.I would actually be very upset if my SIL responded to an email I sent my brother.But that is just me.