I believe that people can love many people all at the same time. I believe that "best friends" are not always a sexual relationship. (But when they are, it is great.)
There is a difference between sex and love. She loves you. The person in South Africa is not "near". "When you can't be with the one you love, love the one you're near." Going to South Africa or having her come there is going to be hard. BUT, if they ever meet, the physical "nearness" of the other person may take away some of the "perfectness" of the love. If not, find a good marriage counselor and discover whether you want to continue the marriage.
Is she "very afraid" of another pregnancy? Is another pregnancy possible. If pregnancy is impossible, drop back to the level of "having sex" and regroup. At some point, you will either have to join in the "love" of the two ladies, or not. If not, you wouldn't be asking the question. So, tell us more, please.
It isn't fair to pretend to ask us our opinion and then surprise us with your answer so we will all look like fools for trying to help.