AITAH for not inviting my sister’s fiancé and his kids to our wedding

My fiancé (M, 27) and I (F, 27) are planning a small wedding. We only invited immediate family and close friends so we can afford to pay for the wedding ourselves. We also said that guests can only bring a plus-one if they’ve been dating for at least a year because we can’t afford to pay for someone they barely know.

Here’s the issue. My sister is 25 and started dating Jon in November. He’s a 43 year old pilot and has two teenagers. Three weeks ago, they announced they were engaged. I’ve tried talking to her a few times about how he’s too old for her and how she’s way too young to be a stepmom to two teens, but she told me to mind my own business and that she loves him.

For our wedding, I only invited my sister. She called and asked why Jon and his kids weren’t invited. I told her it’s because I barely know them and they haven’t even been dating for a year. She said Jon is family now and that his kids will be my niece and nephews, so it’s different. I told her I can’t afford to pay for three extra guests who are basically strangers to us. Now she’s mad and says she won’t come either.

My parents get where I’m coming from, but they also want my sister at the wedding. My dad even offered to cover the cost of Jon and the kids. But the thing is, I just don’t want them there. I feel uncomfortable inviting people I don’t know to my wedding.

Am I being a stubborn asshole?

PS: we only have 24 guests in total . She wanna add 3 strangers to it now

Update : considering my sister just told my mom she is pregnant ( shaking my head ) , I decided to invite them all .

Added later : why I’m inviting everyone ? Because I’m trying to reach a deal with my train wreck of sister ! Bring your crew in return no pregnancy announcement at my wedding ( or any other stunt )