pregnancy is different for every woman (I presume shes pregnant but you didnt actually say).
Some hardly change at all and yet can eat loads, some eat not much more than usual and gain weight, its the luck of the draw Im afraid. Some lucky ones dont even show until last few weeks, some show as soon as a few weeks.
If your sister and her midwife are happy with the weightgain and theres no sign of pregnancy caused diabetes (she will have regular checkups for that as routine) then Im sure alls fine.