The question as phrased, the answer is no. It is not legal. If the 16 year old engages in any sexual conduct with the 13 year old, they could face statutory rape charges and the parental consent assuming there was any would have no bearing

Is it weird for a 15 to date a 13?

No. Your grandson and the 13 year old are both too young to give legal consent to ANY sexual activity. This includes fondling with clothes on.

Is it weird for a 13 to date a 17?

No. It is not illegal for a 13 and 17 year old to date.