My Husband to be has a child with another girl and they have now moved in Jo-Berg but they are coming down for?

Well My husband to be has a child from another girl they have now moved to johannesburg but they are coming down for her birthday in august and they are going to have something for her most likely so i said to him i dont want him spending the hole day with their family i just think its wrong we also got a baby on the way what should i do in a situation like this let him go or what should i do