Birthday -unofficial invites

My twins 3rd B-Day is coming in less than 2 weeks and we’re not doing anything major. My M-I-L has asked what we’re doing and I said it falls on a weekday and whoever likes to drop by they can, we’re doing just cake and drinks. We haven’t announced anything on our family chat, thought it’s best to say it in person to the ones that might be interested or shoot a text. We have no relationship to my S-I-L and if we (husband and I) see each other we don’t talk to at all. It’s awkward and uncomfortable but I’ve come to terms with it and seeing our nature of relationship, I don’t want her over at all so I’m not planning to tell her anything. Is this weird?