alanabelle,I looked up twins and children in The Rulership Book written by Rex E. Bills and this is what it says.
Children in general are ruled by the 5th.
The 1st child is ruled by the 5th.
The 2nd child is ruled by the 7th.
The 3rd child is ruled by the 9th.
The 4th child is ruled by the 11th.
The 5th child is ruled by the 1st.
So if you had twins, I would look to the 5th for sure and then try out using the 7th for the youngest child to see if it was accurate. There doesn't seem to be a hard and fast rule about this, at least I am unable to find one.
Twins as a unit are governed by Mercury and Gemini.
Lee Lehman's rulership book describes children as being governed by the 5th house.