I was in exactly the same situation last year. Dont really know my step sisters as I didnt see my dad much through my teen years when my dad married again. He sort of brought them up as his own cos we wernt around.
When I got invited to the wedding I felt akward as I would only know my dad and step mum.
I did invite them all to my wedding reception but for my dads sake.
I just sent the RSVP back saying I couldnt attend and explained to my dad that I worried being there might pull attention away from the bride. He was cool about it, I think he would of spent the whole day worrying I wasnt being left out or feeling abandened.
I also didnt want to go because I was worried about comparing our weddings and who paid for what. I always say things like I dont care, it doesnt bother me but I knew it would. So in my mind I can picture a small wedding on a tight budget with just a little help from my dad.
Send a card, I'm sure your step sister wont mind and your dad will get over it.
If he says anything about your sister not going, just explain she's a grown up and you dont make her decisions for her.