I used to work in the counseling office at a cemetery. We focused on pre-need planning so that arrangements could be planned in advance so that when someone passed away, their wishes would be carried out.
I know it's not a happy subject to speak about, but you do need to speak to your father in law about it. Some people are superstitious and they think if you talk about these things, you will automatically die the next day or something, but the truth of the matter is, it's the wishes of the person who passed away that need to be respected.
Does Dad already have his funeral property? If Mom has already passed, chances are he took care of that for himself at the same time. What type of service does he want? Is there a special suit or favorite item of clothing he'd like to be dressed in?
Lord knows you don't want to lose him. EVER. But you want to make sure his wishes are carried out.
If he already has his property arrangements set up, you can talk to the cemetery about what is best as far as having his wishes documented. You can also have him talk to his priest.
There are very simple documents that can be filled out. We had booklets for people to put things like that in, like I said, outlining everything right down to what they wanted to be dressed in. Some people want an open casket, some people are very opposed to it. If Dad wants a full catholic mass, he should get one.
Some people actually request no services at all.
It's up to family to see to their wishes, but they can't if they aren't properly expressed and documented in some fashion.
The last thing you want when the time does come, is an argument between your husband and his sister. That's the last thing your father in law would want. Maybe your sister in law was being flip or thought she was being funny, but actually, it does open the door to discussion so that Dad's wishes will be respected and you'll know ahead of time so there can be no room for argument. Unless he's given her the authority to handle it all as she sees fit, what he wants is still quite relevant.
I wish you the best. I know these things can be hard.
It might sound strange, but I loved my job at the cemetery because it was my place to help make these kinds of arrangements BEFORE someone passed away.
It gave the person saying what they wanted peace of mind knowing their wishes would be carried out and it gave the family peace of mind not having to make some of the decisions on top of having lost a loved one.
I wish you the best and I hope your father in law lives at least another 10 years!
Blessings to you and your family!