BeachWater, it's wrong that you feel excluded. However, there may be a middle road.. why don't you organise something really personal and special for the event, however it ends up? Ideas -
a slide show with your mum's family photos on a laptop/
a fabulous celebration cake that you made/
a memories book where all her friends and
family contributed - photos/stories etc
a lovely hairdo at her favourite hairdressers beforehand
book a photographer for the event
You can do all of these, whatever kind of event it is.
Basically, it sounds like you're feeling sad and left out, and your sibling is taking over the show. Is your sister trying to give your mother the celebration the mother wants, or the celebration your sister wants her to have? (iyswim). Give up trying to plan a surprise if it means you both going to war. No party's going to be right if you're going to fall out.
I suggest choosing a venue you and your brother/sis agree on (nice pub?) where thing will be informal but the venue fab. Try and find somewhere where a team of you who are good at food organise a buffet where it's well organised and sumptuous rather than all crisps and fizzy pop. (Outside catered events aren't always that nice, no matter how much you've paid for them.) Formal events are often hideous for surprise parties. If everyone's seated formally, it can be especially difficult, as your mum may not be able to chat to everyone she wants to.
Whatever you choose, try and think of how the event will be for your mum. Remember that numbers swell alarmingly if all invitees bring a friend, so some limitation on numbers will stop it being overwhelming for her.