I suppose it is okay to be polite towards Si. An occasional email to say hi might not be too much, if she wants to send one. I'd be wary of investing much more into it than that. Whatever her motives and whatever happened in the past, it seems apparent that you and she are now traveling on divergent roads; your interests have diverged and your goals in life don't seem to be compatible. She is free to choose her own goals and live her own life.
I see no harm in forgiveness, and I'm sure it will come in time. It just doesn't need to be accompanied by an attempt to re-form close ties; you can be reasonably friendly without trying to pretend that things haven't changed a lot.
Hopefully Si will learn from this and be honest about her intentions in the future. That's up to her now. In the meantime, you have a husband and children and a good balance in your life that seems to be satisfactory to all within your household. As the old saying goes, if it ain't broke, there's no need to fix it. That's my take on it anyway.
Love means never having to say, "Put down that meat cleaver!"