Both you and your sister drew the line as to when you would cut your child off from nursing. She just did it earlier. You're still imposing a time restriction yourself.
It is proven that human milk changes to meet a child's growing needs. There is no cut off when it stops being an excellent and very important source of nutrition. Research has been done anthropologically and it has been determined that the biological MINIMUM for proper nutrition is at least 2.5 years, and can be as old as 6 years, depending on the child's genetics.
Concerning your comfort issue, why is that not as important a thing as nutrition? When did we start placing weights on a child's needs? Not only does human milk provide nutrition, but it also provides immunities, and it has been determined that a child's immune system is not self-functioning until at least three, and more commonly 6 or 7.
So don't complain about your sister. You're doing the same, only later. You're not nursing until your child finishes himself, which will be when he no longer needs the breast.