Well, it might be because she's teething, my uncle and aunt have a son who's teething (and boy does he cry!) they usually rock his cradle and sing the five little ducks song or some other song that he likes and eventually he calms down.
If you can't take a car ride you can put a towel on the washer, put the carseat or carrier on the towel ( so you don't scratch the washer lid & helps from slipping) then turn the washer to spin cycle. The vibration helps calm babies. you have to stay with her CONSTANTLY though in case the carrier slides. you don't want her to fall off the washer!!
Your baby sister may have colic and this causes a child to cry for hours non-stop, for no apparent reason. I have six kids and two of them had colic, it drove the whole family nuts and it was very hard.
We found chamomile tea in the baby's bottle calmed their stomach and stopped their crying. They would often go right to sleep and everyone was relieved. Teething can also cause the baby to cry and teething medicine can help with this.
If these things do not work, your mom needs to take the baby to the doctor, until they find out what is wrong. Good luck.