Are there still things one should never say in a relationship? Yes, most definitely, yes. Whether it’s a romantic relationship, a friendship, a sibling relationship or a parental relationship there are always some things which are better left unsaid.
For example: Never say something along the lines of: you’re just like your mother/father – bad idea it’s just asking for trouble. It’s like throwing a bucket of coldwater on your sex life.
If your friend is involved with a douche, it’s not necessarily a good idea to tell her you think he’s an idiot. She won’t respond well. She obviously see something in him. Plus, it will cause a rift between the two of you – just keep your mouth shut (unless of course he’s the criminal or violent type of douche, then step right in). She’ll probably figure out he’s an idiot all by herself, eventually.
It’s not always a good idea to tell your sibling all about your sex life. They don’t always want to know and probably don’t want to imagine you doing the nasty in all kinds of weird ways.
Don’t tell your Mother that your girlfriend/boyfriend’s mother makes her signiture dish better than she does. She’s your mum, she pushed your big head out of a very small place and it’s just wrong to be disrespecting her like that. Plus it doesn’t help with the bonding of your two families.