4D scan predicament

My partners mum was ever so lovely and has offered to pay for me to have a 4D scan. The only place i can go to is approx 1.5 - 2 hour drive. I am currently not able to drive so my dad said he can give me a lift there and back and i am happy for him and my step mum to come in. Only problem is they are adament they want to bring my little sister who is 4 (they have already stated they can get a baby sitter). I dont want her there, as it will turn into the my little sister show and she will run about and quite frankly be annoying. I love my sister but this is for me and my partner and I and I want it to be special and be all about my little one. My partner is saying i should let her go and compromise because they are giving us a lift there. I quite honestly dont want to be cramped in the back of a car having to pretend to be interested in a 4 year old when lately my patience isnt very high, i've been ill and very ratty.

My question is, am i being unreasonable?? should i let her come?? or should i put my foot down and say no?? also I have no idea how to say no as i have put down a lot of massive hints already but nothing seems to sink in. Or should i just suck it up and hgo along with it all, for a 4D scan??

I am stuck