It's not lily livered to be nervous of the unknown or to run through all the worst case scenarios. In fact, it's probably helpful to do a worst case, what would I do? type thought exercise. It would help you to work out how to minimise the chances of the worst case event happening, like the excellent suggestion of using reins or a Little Life backpack with built in reins mentioned above.
It IS do-able but it wouldn't hurt to plan, prepare and over prepare. Then you can laugh at yourself when you arrive at your sister's and none of your fears came true but you still will have been able to do it, because you know that you can cope even if everything that can go wrong, does because you have A Plan.
I took my eldest 2 DSs on the train to a small festival when they were 3 and just turned 5. It was a 2 hour trip into London, a tube connection across London then another 3 hours on the train out the other side. All that with 2 children, a tent, sleeping bags, pots and pans, food for 4 days, all our clothes and stuff to entertain them on the train. It took a bit of planning and taking some deep breaths before the potential flash points like train changes but it went swimmingly. Apart from the suspicious looks because I was toting huge bags on the Underground less than a week after the 7/7 attacks.
Look at it this way, the actual boarding and disembarking of the trains is a couple of minutes. Why miss out on a lovely couple of days away and a catch up with your sister because of a handful of minutes? Plan how to do it by yourself then if anyone offers to help it is a load lightening wonder. Offers to help tote my bags, hold a child's hand while I got the other on board, lift a child up the steps or stow my bags somewhere so I could get the kids settled in their seats came thick and fast on every stage of my journey. It was actually lovely to see how many people from different walks of life were willing to help, really affirming and reassuring of the innate goodness of people.
For the actual board part, I would suggest going with your idea of traveller packing your bags, then walking together to the door and opening it, before picking up your youngest if she is the bolter, grabbing the folded buggy with the other hand with the reins handle around your wrist and your eldest holding onto the buggy as well. I found "can you help Mummy carry this as its really heavy and you are so strong and such a good helper" to work well for that ;)
Reserve seats on the train. Position yourself on the platform roughly where your carriage will stop. Get on at the door closest to your seat numbers so you don't have to try and wrangle bags and kids down narrow aisles any further than necessary. Once on, move in short hops. Everything in stages. Get to the door. Get on. Get to the baggage compartment. Stow your bags. Get to your seat. Get settled.
You can do it, overwhelming as it seems at first glance.