cultivating a cape gooseberry plant or buy now

If you mean Physalis then they are very easy to grow from the seeds.

They do make very large plants though so not suitable for growing indoors. I had three plants that took up the whole side of an 8foot greenhouse. Grown under glass they die back each winter & come again each year and as Myron says, dead easy to grow - just plant the seeds form supermarket ones. Your sister is lucky hers are still fruiting so late in the season outdoors. You are very unlikely to find plants in a garden centre even in spring.

As Steragram said, they are herbaceous plants so they come back each year, but they probably won't survive our winters unless you grow them in a pot and overwinter them in a conservatory or similar frost-free place; But if it was me, I would grow them from seed and treat them as annuals.

Your sister's plant should be ready for harvesting now. Pick some of the fruits and save some seeds for sowing.

Incidentally, if you leave the brown paper-like lanterns on them and put them in a bowl, they make a nice edible centre table decoration for dinner parties. You can even make a little tree with them using wire and stand them in small vases, or slice the outer lantern at the sides, pull the pieces upwards keeping it attached to the top of the fruits and they look like they have little leaves growing out the top. Great for adding that little extra touch to garnish a desert or even a starter.