Hello and thank you for using this site to help you. Given her age it is very possible that there is canine cognitive dysfunction going on but there is medication to help that.
another possibility is that there is some seizure activity going on. Grand mal seizures are not the only type of seizures a dog can get , there are focal and aura seizures which can have a dog act fearful or overly clinging for hours. This would be something you may want to have looked into by your vet.
Here is more on seizure activity
From a behavior standpoint, it could be that something happened that your dog is associating with your mother in law, not something she did to the dog. This can be as simple as the dog jumped off of something and hurt itself while your mother in law was in the same room. Usually it is something that the dog has not experienced before, it could also be a sound that startled the dog when the two were in the same room. So , in that respect , your mother in law could be the reason, but not the actual cause.
It could also be that she hurt herself while coming back in the door when your mother in law was holding it open and that may be why she will come in only with you.
I would practice getting the dog in and out of the house with you and your mother in law at the door so that the dog can see the effect is still the same with you standing there as well as with your mother in law standing there.
If possible, put the dog on a leash and have them both walk in together as well.