Your sister is right. A four year old tree loaded with fruit should be culled. If you don't there is a good chance a strong wind will come along and break or twist the young branches setting the tree back a season or two. The fruit could also drop before ripening, leaving you to pick it then or leave it to attract fruit flies and vermin.

I agree. Some varieties need thinning in order to prevent biennial bearing.

All pears - regardless of age - should be thinned!! Otherwise the fruit is unable to develop fully and as noted, the cropping could be heavy enough to break branches. You want to thin to 1-2 pears per cluster and the clusters should be at least 4-6 inches apart.

Apples, plums, peaches and apricots should be thinned as well, although apples generally require more thinning than other types of fruit trees.

It should be noted that some varieties of pears are far more likely to over-fruit than others.