Maybe if you put it to SIL like that she will understand.
It needs to come from her own brother, not from 'bridezilla'.
Her brother sends this text:
"Chloe, I just wanted to talk to you about the wedding again. You are my little sister and it would mean so much for you to be there when Gina and I get married. Mum and Dad have said that they would pay for your room if you want to stay in Dundee overnight, but as it is less than an hour from your house you could make the journey back home afterwards if you preferred. I know you were worried you wouldn't know anyone, but just to remind you, there will be people you know. Of course you know Mum and Dad, Uncle Charlie and Moira, Grandma, Sean, Jim, Cathy and Raj. (List them all, so she can see the 'I won't know anyone' is BS). And hopefully you will get a chance to get to know some of the other important people in our lives.
I just wanted to remind you that we are on a very limited budget, and we wanted an intimate ceremony with only those people we love there. That means we each have invited ten friends, and none of those friends got a plus one. Some of the friends are couples, but we only invited both people if we know and love them both. I know you wanted Steve there, but the total number of guests is 30, and this number includes both families. If Steve comes, it means someone we love will be unable to attend.
We understand Steve is important in your life and we would love to get to know him. We get back from honeymoon on X date, and are back at work that week, so the following weekend, Saturday the third, we would love to take you and Steve out for dinner and get to know him better. You will be our first meal out as a married couple!
Please give me a call when you're free."