Guppy? I think you mean a 'tadpole' trike? Tadpole configuration refers to 2 wheels in front. I can't offer a lot of help. Different trikes have different track widths, and front wheel size has an effect on how much the front wheels impinge into the cockpit when turning. I wouldn't recommend changing wheel sizes on an existing trike - doing so may affect the handling in a negative way. Which is all to say that your trike may not work for your sister, she may have to get one of her own - one that has a wider track.