You say that she has nieces and nephews on your side.
See, this is just another complication. Presumably these are your sister's children ? Your sister wants no contact with your half sibling so you simply don't have the right to discuss her family with the half sibling.
Balancing everyone's needs and wants is impossible without hurting someone.
This woman was abandoned by your father, presumably was part of a hugely difficult time in your mother's life and she missed out on sibling relationships. This is t going to be some bittersweet reunion..
Someone is going to get hurt and at least one person is going to feel rejected or betrayed. In the real world without the bonds of shared memories and experiences, your half sibling isnt part of your family group and you could lose everyone. You need to consider what you hope to get out of this possible contact. How will she feel if you initiate contact and then don't want to take it any further ? How will you feel if she decides not to have contact with you?
You might gain a sister but you might not and you could damage your relationship with your family in the process.
I feel for you as only you can decide. Maybe it would be worth explaining to your mother and sister how you feel and see if you can do this in a way that they can tolerate. If not and you still want to go ahead, you need to accept that it will have consequences for all of you.
Like I said before, it's just so unfair that you're all in this position.