First of all, it's none of SIL's business.
Secondly, nursing is NATURAL and NORMAL at age 2 (and 3 and 4 and...) and provides the nursling (and the mama!) with much more than calories/nutrition (antibodies, reduced risk of getting certain cancers and other diseases, higher IQ than formula-fed counterparts, etc. etc. etc.)
Third, nursing helps prevent obesity.
The 22 month old is getting mamamilk that is especially formulated for her. The fat content is what she needs at 22 months (as are the other nutrients). Mamamilk changes as the nursling grows...it doesn't stay "put-on-weight newborn milk" as many people assume. If anything would cause a 22 month old to be "overweight" (being "chubby" at 22 months old is normal!) it would more likely be the solids she's eating!
That said, there is one case I know if where nursing can put a few extra pounds on a toddler...when mama nurses said toddler through pregnancy and then tandem nurses. In that case, the toddler is getting mamamilk formulated for the baby (higher in fat, etc.) and can put on a little more weight. This isn't a problem, as once the child cuts back on nursing or weans, the weight usually comes right off. (I have personal experience with this, as I'm nursing through pregnancy for the third time / tandem nursing through pregnancy for the second time. My "weaner" is the weight he "should" be at this point, although he was high on the weight charts until he weaned.)