MY dog hates my MIL

We (me, my husband, 2 kids under 13) rescued a mixed breed dog 2.5 years ago. Other than one incident with my oldest son on our first afternoon after bringing him home, our dog, Harvey (4), has been pretty great. He's responding to training, he loves our family and our home, and we love him dearly! The problem is that he HATES my mother in law! She is an older woman (80), a little unsteady on her feet, and she has recently moved to be closer to us. Harvey barks at her non-stop and often charges her. One time, she sat down on the couch and he ran right toward her, jumped up on the ottoman, and she reached out and hit him away from her. Although we don't condone Harvey acting like this, I also told MIL my that we DO NOT hit him. We don't know much about the life he led prior to being surrendered to the shelter, but he had some health issues and probably wasn't loved and taken care of as he should have been. The behavior has only gotten worse to the point where MIL doesn't usually come inside our house and will just drop things off or pick things up in the driveway. We can kennel him, but he still barks so hard that he stinks up the room. We hosted Christmas at our house this year and attempted to board Harvey for 2 days. Unfortunately, the boarder was full. We have some prescribed anti-anxiety meds for him and decided to dose him so he could be relaxed while she was here. We also made sure he had been exercised and did everything we could think of to be proactive. When we allowed Harvey to be out of his kennel he was constantly trying to be near MIL. Every time he approached her, my husband intervened and sent him off to some other family member that would give him attention and pets. However, towards the end of the evening he walked right up to her in her chair and bit her hand. He didn't growl or bark. He didn't even try to hurt her as it wasn't a painful bite. He definitely could have hurt her if he wanted to. I am confused because he doesn't act this way to anyone else. He doesn't bite or nip. He only barks at people he can't see outside our door. She really isn't mean to him other than the previously mentioned issue that happened almost 2 years ago. Why does he do this with her only?