Grandparents Claim Against Sign Language

We are having an extremely difficult time with my parents (my mother and stepfather).
I have a 6 year-old with severe verbal apraxia and a nearly 3 year-old with mild apraxia (?)/phonological disorder. We use SIM-COM (simultaneous verbal communication accompanied with Signed English because we want them to learn English Syntax) in order to communicate effectively with both sons (6 year-old also has a 40dB hearing loss); otherwise, we would be unable to understand most of what either boys are saying to us. The sign language is of great help to all of us, but I think my parents get agitated because "they" don't know sign language, as they boldly confess, and they do not choose to learn it either.
My parents are caught frequently saying things to my boys (when they verbalize and do signs) like, "I don't understand sign language; you are just going to have to speak to me (all the while, my boys "are" verbalizing, but unintelligibly, like a stroke patient." And recently, one of my older girls was in tears and when I asked what was wrong, she stated that my stepfather saw/heard her communicating with my son (her brother)and he scolded her and told her, "He'll talk if you just stop doing that (signing). You need to stop that because I don't understand it and he "can" talk." (when clearly he cannot speak intelligbly). In addition, my parents will (in the wrong way, opposite to what we learn in speech therapy) try to get my boys to speak "correctly" (by moving their mouths a few inches from their faces and making drawn out sounds, and then saying "that is right" when they say something completely wrong- wrong sound, wrong mouth posture, etc.. Well, you get the picture.), all the while the boys are groping for the correct words/sounds.
Point is, they "believe" we are keeping them both from being able to speak "intelligibly" by us using mands and verbal speech both, which I think we all know is not the case. And they boldly say so as well. (Very hurtful, but they do not care.)
They truly do not understand apraxia and when we try to explain, then they do not want to hear it and either hang up the phone (offended), leave (if they are in our presence) , or quite frankly they become really rude (and insistent that if we just "make" them talk intelligibly, they will). I will even say they are fanatic in saying that we are speaking bad things over them and we should just profess they "can" talk and then they will.?(I am all for the power of positive words, but they do this in an effort of denial that a problem even exists at all. :(
It is a hopeless situation and we cannot abandon them BECAUSE we rent a house from them and really have no other optins or $$$ to do otherwise. We are stuck with them, but really do not like them reprimanding ANY of our children, making them feel bad or demanding that the boys "just speak" when they physically cannot do their request on demand. This is causing more tension and more accusations from my parents that we are "turning the kids against them" and "quizzing what is said to them when not in our presence (which is what they accused me of doing when my daugter voluntarily brought it up to me about her being scolded when I asked why she was crying)"... (a totally unwarranted and untrue statement, as usual)
Any suggestions on how to handle this? This is not fair for them to basically mistreat us all in this manner and frankly, I am tired of trying to get them to understand, BUT me giving up on trying to get them to understand is not going to resolve the problem, which is their shunning the sign language and putting us all down for employing it as a bridge to communication. I do not need anyone making my children feel "bad" about communicating the only way they can, do I?
Exasperated,
Susan