My sister pregnant to a guy who is seperated but not divorced...he wasnt up front....?

You probably should just give her space and let her make her own decisions and just do your own thing, and stay objective to her situations. It seems like more drama than it is worth, and not worth any future aggravation or headaches. She is old enough to make choices, whether they are right or wrong. It is probably best to not be in the middle of this.