A.M.
C.,
I am sorry you are having problems with your family. There are so many layers here that it is impossible to address them all in this format.
First and most importantly you and your children cannot get herpes (genital like your sister has)from her in a casual setting. Genital herpes can only be spread through sexual contact. Yes, genital herpes can spread to the mouth but only through oral sex. As this is highly unlikely in your case, you have nothing to worry about. Oral herpes is an entirely different strain. 65% of the population has oral herpes. If you get cold sores they are most likely oral herpes. Both strains need to be spread by direct contact. They will not spread through sharing cups, straws, etc. To reiterate genital herpes can only be spread through direct sexual contact.
OK, on to your sister. The choice is yours but she is definitely trying to reach out to you. She is pregnant and looking for support. You can choose to support her or not. I know it is difficult to let go of the past especially since it repeats itself. I have had this problem too. I really suggest you talk to a counselor about your family situation. They can help you sort through all of your feelings and figure out what to do in the future. If you decide to terminate all contact you will know why you are doing it. If you decide to establish a relationship you will learn how to control it. I have a similiar situation and after many years I have a good relationship with the problem members of my family. The key was learning how to control the negativity and counseling gave me the answer.
As for the party just tell her it is "kids only" - you hope she understands and you can get together (or not) at another time. Then make the party kids only. This is the best way to avoid difficult family situations. Of course next year she will have a child so you will need to really figure out your relationship by then.
Good luck. But remember toxic relationships poison your entire life.
A.