My husband loves our two daughters very much...but the thing is, he's not their father. Should I tell him?

Ultimately it is up to you. No matter what you choose to do, it will affect your relationship. Consider the scenarios and decide how you want to handle this. For example, not telling him means you will likely be living in fear and guilt that one day he will discover the truth on his own. Or, if you choose to tell him, be prepare to give him time to process. Explain and keep the communication open. Seek external professional help (i.e. marriage counselling) if both are open to it.