"My sister has stayed friends with her - her statement being that the OW was like her other sister for years"
Well, it clearly suits your sister to say that, because it excuses her decision to remain close friends with the OW ... but ... it's clearly not true, otherwise she would be asking "her other sister" to be her Bridesmaid too.
However that wouldn't suit your sister, that might really spoil her wedding day, so instead she is making a choice that will only spoil your time at her wedding day ... worse, on some level she knows that, but is refusing to allow it to matter.
Actually, (and very weirdly
) I am furious with your sister on your behalf ... but I get that you love her and want to be there for her.
If you really feel you have to go the wedding ... if you do want to go, albeit for your sister.
I think you should tell her all that. I think you should tell her that this will be incredibly difficult for you, but you love her and want to support her, so you will be there.
Tell her that you will not speak to OW ... won't ever acknowledge her presence ... and you will do everything within your power to tolerate the intolerable, for your sisters sake.
Tell her that you will be her Bridesmaid, if that is what she wants, but you will understand if she wants to rethink that, if she is concerned that you being bridesmaid under such circumstances (e.g. you not speaking to OW / you not acknowledging OW's presence /you having to tolerate OW presence) will put an atmosphere on everything.
Tell her you will do all that, because you love her, and you will hope the wedding crowd will help you keep a low profile and get through the day ... but tell her that it would, for you, be an impossible step to also attend a hen do, to which OW was a guest, so you won't be going to the hen do.
Once you have told her all that (even if you have to write her a note) her reaction will tell you exactly where she stands in this. A sister who cares about her sibling would un-invite the OW.
p.s. Of course, If you don't really feel you have to go the wedding ... if you can't bear to go through with this ... then you really should not go.