When you use My family with verb to be in singular, you're referring to your family as a group or entity. I.e: my family is in the US. it means that all of your family is there, living together in the same house.
If you use the verb to be in plural in this case, you're talking about all of your family members separately. so if you say "my family are in the US", you mean that all of your family members are in the US, but they can be living in thifferent houses, neighborhoods or cities.