Family unity is very important, especially if you want to have an influence later in their marriage and even more important when they begin to have children. You do not want to shut out your family. We are not Amish. We don’t shun people.
There is plenty of information from the OP that makes this decision easy. If you and your family did not practice any sort of religion while you were growing up then your decision to not go would be incredibly hurtful and jolting. They are not used to this sort of mindset. They wouldn’t understand. It would only cause to alienate them from your life or live with this ongoing resentment.
Don’t risk breaking up the family. Depending on what the relationship is between the sisters, you could talk about the struggles you are having with this. If that might hurt her as well, then don’t say anything. Be happy for her, supportive and loving. In time, you might find yourself taking her children to Church with you or even your sister and brother-inlaw.
And please no lectures from posters about how the faith and truth can be hurtful and that’s just the way it is