weird husband and unreasonable MIL

Hi all, need ur opinion 1. My husband said that can wash baby teeth with adult toothpaste. Is it true? He said we can save money on that. 2. My MIL keep wanting my son to play or "be best buddy" with my husband's cousin son. They are at same age that is 1 year old now. My MIL will buy twin cloth, etc for them. Just because my husband n his cousin are best buddy, they hope their son can be the same. What should i do? What if my son doesnt like this arrangement? Why must force him? 3. Should i stay or go away from this weird family?