Yes, children/teenagers could say don't be a sissy (don't be a coward) but it also has a sexual reference which is offensive to some.

Yes, definitely.

Yes. It's also grossly sexist. Don't use it.

Yes, usually, particuarly when addressed to a boy or man. Sometimes, though, a sister might be called "sis"or "sissy", and then this is just a normal, affectionate nickname. There was a very well known actress, Sissy Spacek. There is apparently a particular subculture where the "girlish" meaning for a man is regarded not as an insult but a compliment, but I have no direct experience of that. https://charlies-names.com/en/sissy/