Does she know what she's doing though? Is this a behaviour that she's always had, or is it a bit new? Might it have developed over time, and just hasn't been noticed or previously put down to just her personality?
If yes - if it does seem new, I'd consider chatting with your parents and seeing if it might be worth seeing a GP or something.
She might just be lonely as well (and we all have needs, whether we like to think about it or not - maybe that "need" isn't being fulfilled). Loneliness can be dealt with as a family (social activities, voluntary work, etc) but sexual frustration would really need extra help, it's beyond what you can help with and not really the role of your parents either.
Whether it's new or not, medical or not, I'd try first pointing it out firmly and from then on, ignoring and distraction.
E.g. Granny mentions that her husband's trying Viagra - "OK Gran. Now, what have you planned for Sunday? We thought about taking the dog for a walk.." etc.
Avoid giving her the chance to rub your husband at all - have him sit next to you, her on the other sofa, he goes to make the coffee and she stays with you (where possible).
I would give the lifts home with just him and her a miss if possible, even if both of you have to go.. I wouldn't be jumping at sending her on the bus or something (she sounds a bit vulnerable), but perhaps given how pregnant you are, you could suggest that the regular visits to yours stop soon (maybe you're too tired? need rest?), to be started up maybe once the baby's born? Or if possible arrange only to see her at your parents' and then they can deal with the lifts home?
You're absolutely not unreasonable - sounds a very uncomfortable situation all round, but I would consider the fact that there might be other reasons for this behaviour beyond just being vindictive/selfish, however much it seems like she is.