I work with seniors in their homes and I have recently started with a new client in a faciity that is well known in our area.
If the grandmother is now in a full time nursing facility coming from the assisted side, then I can tell you, she needs 24/7 coverage. When you enter a full time facility, there are reasons for doing so.
I can understand FIL's comments about not being able to enjoy the party if his mother was there. While all of us would like our family to be in attendance, unless you have been with certain patients, (not knowing grandma's condition), you really don't know what you are dealing with.
My new client, I am only with two hours for companionship, but yesterday, I spent most of those two hours walking halls an such because she was so confused and could not seem to settle down.
Yes, I am sure you would like to invite grandma, (and to me, as long as the ones doing the inviting do not care about possible outbursts, etc.,) then it is really no one else's business), but my question to you is, "Who will be responsble for grandma?". Is it usually up to her four children that take on this responsibility? If so, then I would be asking each of those children if they are willing to take on that responsibility and if not, if your DIL and DS or whomever is willing, would the children mind if a full time caregiver was provided for the event if grandma would like to and is able to attend the event?
You have already stated you will not be able to. Your children will not be able to, because it is their event. So, by inviting grandma, her care is going to fall on others and those others are the ones who should be consulted about grandma attending the event.
Also, others have stated, send the invite to her and see what she wants to do. I would really try and inquire as to her health, if she is able to come. Alot of times, depending on their state of confusion, too many people/noise might agitate them and cause them distress. Also, most incontinence issues are no problem, they can be dealt with by weaing depends, but I have had clients who when have made a bowel movement, tend to play with said bowel movement.
There are many issues here that need to be addressed before sending an invite directly to grandma.