It's possible that this is happening with your mother ... because she's your mother. As such, it may be all but impossible to affect the desired outcome by simply posing the invitation differently – at least as far as your mother is concerned. I've provided a little advice about that below, but first I'll address the question you actually asked.
Saying it like this should not make anyone feel obligated:
We're holding a little impromptu/informal gathering [when]. If it fits your schedule, feel free to stop by!
But if I'm not mistaken, that's basically the way you're saying it already. It's polite, inclusive, and in no way makes it sound mandatory. However, if you feel you need to be more clear about there being no obligation, then that's exactly what you should do. After announcing the event and extending the invitation in your normal, informative way, try adding something to clarify that point directly:
To be clear, my family will be doing this even if no one else shows up, and we'll be happy either way. I'm just letting you know it's an Open Door event.
Hopefully, that will help curb your extraordinary appeal!
In the case of your mother, it's possible that it's just her love for you (and her personality) that's causing this phenomenon. It might be best to just accept that she'd prefer to put you first and be thankful for that – especially given that hundreds of millions of people are not so fortunate. But if you do choose to address the matter, it may require more of a heart-to-heart than a rewording of your invitation:
Something has been on my mind, Mom. I've been trying to figure out a way to phrase my invitations so you won't feel obligated to come. I love that you take every opportunity to put me first, of course, but I know that sometimes you even cancel your own plans to do so. That makes me a little uncomfortable; I want you to have your own social life, too. Can we maybe do this – I'll make you a promise if you make one for me?
I promise to never say that an event I invite you to is "no big deal," and that it won't bother me one bit if you can't make it, if either of those things aren't true.
In return, you promise me that you'll hear me and believe me when I do say that and that you'll try to enjoy yourself in ways aside from spending time with your favorite son! LOL
OK. Good luck!