Reddit reviews The Great Big Book of Families

"There are lots of different kinds of families - some have just a mom, some have just a dad, some have a mom and a dad, some have two moms or two dads."

If they continue asking questions, she can add something like, "You had a father but he wasn't ready to be a dad, so that's why he isn't here. Sometimes that happens, but it's okay because our family is wonderful just the way we are!"

There are some great picture books that help normalize different family structures as well, such as The Family Book and The Great Big Book of Families. There are also a few books that specifically celebrate single-parent families, such as Two is Enough and Love is a Family (although they may be less applicable for twins, since they show a mother and a single child). If the kids have any classmates being raised by single parents, you can point that out too.

I've never been able to find anything specific that is a good fit, but I do like these books that explain that there are different kinds of families:

http://www.amazon.com/Family-Book-Todd-Parr/dp/0316070408

https://www.amazon.com/gp/aw/d/1847805876/ref=sxl1?qid=1465489682&sr=1&pi=AC_SX236_SY340_QL65