I have little technical knowledge or advice, but I do know the subject and the shooter.
Your mom has given you a real challenge.
Number one, use a tripod!
Burn some film and see what happens. Take notes so you can repeat what works. Since she's your sister, take advantage of your frequent access by experimenting with locations and light and then go back and re-shoot what, where, and when it works best. The trees behind your folk's house might give good open shade in morning and evening, probably good for face shots. You might get a nice look under the trees at Lake Marion beach with the lake in the background, good for full body shots. It will probably be empty in the morning. Try using your flash for fill in light.
Does Melissa remember where the guy shot her sitting by a stream? That was beautiful.
Take your film somewhere that offers quality processing and printing. National Camera by Burnsville Center might be a good place to start. Maybe somebody from the Twin Cities can chime in.
Start doing this right away so they have time to go to a pro if it doesn't work out.