Need advice regarding coming out to religious grandparents

Hello all let me first just thank you for taking time of your day to look over this post, this has been making me stressed for some time and I really need advice.I have came out about a year and a half ago and all my friends know and so does my mum. I'm currently living with my boyfriend who I have been seeing for just over a year now. I'm originally from Portugal and have been living in the UK for the last 12 years of my life.The problem I'm faced with is weather or not to tell my grandparents I'm gay. Here are some things to consider. My grandparents are jehovas witnesses or how ever u spell that. Their old and well stuck in their ways. They were the ones the brought me up for many years as my mum unfortunately didn't have the conditions for me to be with her until a later age. My grandparents are great people I love to bits and in many ways I see them as my parents. Usually I go back every year for holiday but it's now been some time .. 3 years to be exact since I have been and been getting quite homesick. On top of that there nothing more I would love to do than to take my boyfriend to my home country for a holiday and for him to get to meet my family over there. I really can't stress enough how religious my grandparents are and that follows for a big chunk of the family too.I think if I was to tell them they wouldn't accept me for a while but would eventually come round. Maybe not fully I think our relationship would deteriorate in some degree but I know they love me and probably that would be the outcome.But I think that the initial shock could be potentially disastrous and say one of them has a heart attack (baring in mind the extremely religious family, their age and health issues). And if that wasn't enough I also think that even if that didn't happen the stress and hassle I would put them throgh at that age between them and the rest of the family (this is hoping they eventually accept it). There's nothing I would of liked more than to tell them however I really do feel like it's not the right thing to do in this case. I would rather wait for them to pass however bad that sounds and then I won't care who finds out. I really couldn't have neither of heir deaths in my conscious or the fact of knowing I have placed a considerable amount of stress in their shoulders at that age with endless arguments and falling outs between them and the rest of the family.My boyfriend thinks I should tell them which is why I'm making this post in the hope that I will know more certainly if I am indeed doing the right thing. Also keep in mind yes I'm aware and I completely agree that if someone can't accept who you are they're not worth it. However this is not the case and there's a lot more at play here and that's what's scary :( I don't want to play with people's life's specially the ones who I see as my parents.