at 18 weeks she could well have felt baby move.
But baby is so small that generally a lot of the movement wont be felt, the same if baby turns and is kicking her back.
Until 25 weeks or so movement wont be egular at all and to feel it mum needs to be laying down, still and quiet for quite a while at times.
My two were both most energetic when i was asleep according to Mr CRX. They would both kick all night and I'd sleep through it.
The middle part of my pregnancies were also when i felt the least pregnant, the exhaustion had worn off and i had energy and felt the same as i did before pregnancy.
chances are, because of those reasons, your sister is worrying over nothing, but she could call her midwife, they usually give out their mobile numbers or call the surgery, and they will advise her.