Tell the cops to hook 'em both up. Its a very serious accusation, and the complete truth needs to see the light of the day. Anything less and there would always be questions.
In a sickeningly high number of cases the mother doesn't know, doesn't want to know, knows and wants to cover it up out of fear, or knows and likes it going on for some sick reason; so you can see why they might not want you there during the questioning.
Be glad your husband is even given a chance to clear his name. There have a frightening number of times were the man get railroaded and has no chance at all.