Being on the phone with all your friends and holding secret conversations with your private jokes outside of parental supervision is typical teen behavior and she’s entering that phase.
If the mother and sister decide to talk ill about you with the child, there really isn’t much you can do. If the child doesn’t treat you badly or disrespect you, and if you don’t feel that her conversations are harmful to her (i.e. sexual predators, engaging in dangerous behavior, etc) then let it go. It’s not a battle to fight at this point. Spying on her will only reinforce anything negative that her mother/sister MIGHT be saying about you.
Unless the girl is reacting to negative information, why should you care what her mother thinks/says? If the child is treating you ill, however, your husband should address that issue on its own and needs to without resorting to invading her privacy.
Keep in mind a couple of things about this. 1.) She may be aware that you have seen her messages and doesn’t like the invasion of privacy. 2.) She is becoming the age where privacy becomes much more important, so she may just naturally be deleting her text msgs because she doesn’t want anyone in on her “secret conversations”. 3.) Text messages take up space and with the amount of texting you are describing, she’s most likely having to delete a fair amount on a regular basis anyhow just to make room for any incoming messages.
Keep the lines of communication open and keep your eyes open for any bad/dangerous situations. Invading her privacy too soon and without good cause could break any trust she might develop in you.