I would keep the front door locked and just not open it to her. 

She is marking "her" territory. Lock the door and yell at SD that her mom is outside waiting for her, loud enough so BM knows the door is locked on purpose. The BM in my life eventually went back to her vehicle to wait and never tried that shit again. 

We had a glass screen door, so I glanced from washing dishes and yelled. 

Why isn't the door locked?? Unacceptable. Bm tried this ONCE when we were living in the same town until I layed down the law. Pickups at curb only.

Next time she tries this stunt, make sure the door is closed. If, by any chance it isn't, pick up the phone, call the police and tell them you have a tresspasser on your premises and that you are scared. She won't do it ever again.

Your DH needs to send her a message letting her know that she is expected to text ahead and wait in her car, or whatever it is she does when he's there.  Or whatever is acceptable to you - if you are okay with her coming at a certain time and ringing the doorbell, fine. If you want her to text you, fine. If you want her to text him and he texts you, that's fine too. Whatever works - but walking right into your house is unacceptable.

And meanwhile, lock the doors. This is a power play on her part, and it's YOUR home, stop letting her disrespect you. And if your DH won't set those boundaries with her, stop watching SD on his time.

I'm one of the few that doesn't mind BM darkening my doorway (because she tends to spill the beans on everything happening in her life so we know what crazy BS is happening), but her entering my home without knocking and being invited in would be a bridge too far.

Locks on the door and a conversation from your DH would be a must. Depending on your SK's age, I'd have them sit at the front window with their stuff at the first text so they can greet BM on the porch or in the front entryway (or at the car, whatever you're comfortable with). This way, BM can't complain that it "takes too long" for SK to get their things together and use that as an excuse to try to come inside and snoop.

I'm curious, was this her marital home with your DH? If so, were the locks changed? If not, I highly suggest changing them, and perhaps changing them to a keypad entry or a key fob entry.

Finally, there is always the options of your DH being there when SD is being picked up, or he can drop SD off before he goes wherever he is going. 

As soon as she texts, luck the door,  deadbolt it, and use the top chain thing. When she asks why the door was locked, let her know, she can't just be waltzing all up into YOUR home.