As-salamu alaykum,
Thumb sucking is common is children. While many parents do not like it, it is often a habit children drop as they grow older. Kind reminders are helpful, although they may also serve to reinforce the habit as children may start associating thumb sucking with parental interaction. I would kindly suggest sister that at this age, you do not make a big deal of it and reward her with little things when she is not sucking her thumb. Also when she is suckling her thumb replace her thumb-sucking with a soft toy or favorite blanket as thumb-sucking is often a way children self-sooth or relax themselves. The less you make a big deal about it, the sooner she will forget about this habit and transition out of this habit.