Your sister can obtain a dispensation from Catholic form from her Bishop. They will have to go through Catholic marriage preparation and be determined free to marry-- which may involve a decree of nullity on his part. Not enough details to know what will be involved
If they go through proper channels and receive proper dispensations and permissions, her marriage will be perfectly valid. And, in such a case there is no moral reason you cannot attend. Only if she chooses to marry without proper dispensation or permission would you have a moral reason not to attend.
You seem to be putting the cart before the horse. They are not engaged. You do not know her plans for marriage, yet you assume that she would violate Church teaching and marry outside the Church.
How about talking to her about it instead of taking this attitude before she’s even engaged?