I'm not going to tell my depressed pregnant sister all the lovely things her boyfriend thinks she needs to hear
You can't really do that. Why are you keeping it from her?
I think it's a matter of time before she figures out that she is less important to him than the other girl.
Relationships can be complicated but you should stay out it. It could get worse if you interfere and make your sister suffer.
Who cares. I don't want her to have high standards because she wouldn't accept anything less when it comes to love.
I think it's more wise to just tell her everything she needs to know, she doesn't deserve to be kept in the dark. Sometimes it's best to protect them even if it will hurt them in the end. Perhaps you can tell her now that he is cheating, so she doesn't have to be more devastated later?