Dinner and Sister?

[SoftKittyWarmKitty11]

I need some advice, my DM's birthday coming up I suggested my parents come over for dinner just them, me and DH the evening before her birthday. However my DSis has messaged me asking where her invite was, she still lives with my parents but am I in the wrong to say I was planning to just have us 4 there because they've helped me and DH out loads recently so we wanted to do something nice? I'm a really non confrontational kind of person and I know if I tell my sister no she will be pretty annoyed at me. She is older than me so it's not like she's a teenager who will be home alone or anything. I just don't know whether I should just let her tag along or stand my ground.... if I'm in the wrong here then I'll hold my hands up but just wondered if anyone had any thoughts?

Well I get on really well with my sister so if she was around I would always invite her, but I guess I’d start from a standpoint of what would your parents want as it’s their treat?

(As in a treat for them)

I do get on really well with her so it's not like I hate the idea of her here too, we'll when we all spoke about it, it was the plan for it to be the 4 of us.. I hadn't thought about asking them if I'm honest as I went into panic mode when she messaged as I hate to upset anyone especially my family! I'll ask my DM, if she's happy either way then it's all good! Thank you, I just needed someone a bit more level headed than me to point out something obvious that I hadn't thought of