I think its important info that your MIL has had strokes...she IS brain-damaged (and i'm speaking from experience, my mother is a stroke survivor as well, and sometimes responds inappropriately to things, such as laughing if something sad or scary is on tv)....so....how would you deal with this situation, if it were another child who was treating your son this way? Or what if it was an older sib of your dh, who was developmentally disabled? Would you limit your choices to "Do nothing, and hope dh says something" or "help ds be nicer"?
What you need to do, the next time your MIL behaves inappropriately, stop her behavior. Put a hand on her shoulder, or lightly touch her hand or whatever it takes, and say (while directly looking at her)"MIL....ds does NOT like it when you poke him, please stop." Also, i would encourage you to set up some playtime situations with MIL and ds, where they can interact appropriately (a simple board game, a game of catch, a walk around the block....whatever), and where you can be there to sort of watch the interaction and intervene if necessary.
Also, if your ds is three, i think he is old enough to understand brain damage. My son had just turned three when my mom had her major stroke, which left her in a wheelchair and unable to use her right side, as well as affecting her speech so that she can barely talk because she can't produce the right word for what she is thinking (aphasia)...it was quite a change. I told him that Grandma had hurt her head, and it damaged her brain, and the part of her brain that controlled certain things (walking, for example)was damaged. That she might act a little bit differently, and have problems walking and talking but the grandma that loved him and took care of him was still there. I think its important that your ds knows that grandma isnt *meaning* to be "mean" or irritating, that the part of her brain that would stop her from doing those things has been *damaged*, and she needs help working on responding differently. Perhaps you can get everyone on the same side, of helping Grandma know when she is being irritating. Tell him, when Grandma is bothering him, to immediately come get you, so you can help him handle it.
I dont think its ok that your ds is "mean" or "rude" to MIL, but i understand his frustrations. I think part of what you need to do is help him express himself in a way that is not disrespectful, even if that means being there to say to MIL "Ds is upset that you are poking him, that is why he is yelling at you. Please stop. Would you like to play this game with him instead?" If you can't be there to intervene, perhaps you need to limit their alone time together, if possible (meaning, grandma doesnt sit in back with ds, for example, but sits up front w/ you.)
I also don't think that its inappropriate to discuss with MIL the affect her strokes have had on her behavior, and how she may not realize how irritating she can be to ds---in fact, i think its imperative in this situation. I think its pretty sad that dh won't just SAY that to his mother, instead of "gosh i dont know"...how can she possibly work on changing her behavior if she isnt aware of exactly what she needs to change? Are there other issues with Dh and his MIL, that would make him not want to be upfront and honest w/ his mom?
I've worked with people who have TBI (traumatic brain injury)as well as stroke (and mental illness, and mental retardation, autism, etc)....you need to be clear and consistent with your MIL, to protect your ds from her behavior, and honestly to stop behavior from your ds that is causing MIL hurt feelings as well.
Katherine, single homeschooling mom to Boy Genius (17) Thing One (6) and Thing Two (6) and one outgoing Girl (12) and hoping for more through foster care and adoption