When is a good time to ask about an estate?

It's never too early. I vaguely know where my parents' paperwork is although right now, my aunt is the executor. I do know they have updated it to factor in a certain amount and things for the boys. Otherwise, it's 50/50 between me and my sister. If something happens to my aunt or if the relationship between Dad and her gets worse, I'm sure they'll update to make me or DH the executor. Yes, they'll make DH the executor simply so that he can deal with the phone calls I hate. If your MIL is retired and is drawing social security, then I'd tell DH he needs to at least find out where the important paperwork is.

I have spoken in vague terms with our son.
I have one son and 2 step sons. When we redid our wills a couple of years ago, I was going to tell all 3 all of the facts. Our attorney advised against it. He said it creates problems.
Most of our assets will be split 3 ways. However, my son will get more $$ because it is money from my family. I have explained that in a letter to be given to the sons upon our death.

I have told my son that xxx will be yours upon our death.

It's never too early while they are healthy. I will say a tacky time is to hand out a piece of paper at the graveside stating all assets DH's grandmother had will go to grandmother's DD. every family member got this paper handed out by DH's aunt's husband. It never crossed DH's mind about her assets and especially at her graveside service.

My mom and my ILs each gave us a paper that has the location of all important papers. I don't know what's actually in their wills, but I know where the copies are and who their lawyer is.

I think you should ask that if anything happens to her, where does she keep important papers. You might want to ask her if she has a health care proxy in place, also. Having my dad's proxy made things much easier when he had advanced Alzheimer's and lots of decisions needed to be made.

I would just leave it alone. If she asked for your children's SSN#'s, then I would think she is leaving her estate to them. If you have the kind of relationship with your MIL to where you can talk to her about it, then by all means talk to her about it... but if you don't, then what else can you do?

My mother's only asset is her house and although we haven't had the discussion, I suspect it will be split between my brother and I who are her only biological children. My other siblings will get certain possessions, particularly those that originally belonged to their mother (china, some jewelry) but I never would have begrudged them these items anyway. I don't believe they have any interest in the house, it is run down and not worth much.

My DH has had the discussion with his mother and stepfather. His step sister is the executor of the estate and they know their assets will be mostly liquidated and split evenly amongst the siblings. Our share is either ×× amount of cash, or their current house plus cash to make everything even. I know I will be receiving MIL'S jewelry collection, DH will receive his stepfather's car. Everything is spelled out in writing and we have no worries about his sister making things right.

The only problem is DHs brother, who is a greedy, angry, intolerable ass. My biggest fear is that when they finally do pass he will swoop in and start taking things without regards to the will, or that if either in-law passes to before the other he will move in and take "possession" of the house and everything due DH and I. Once that happens it will become an intolerable fight to make things right, he will never give up what he takes, it may come to blows between him and DH and it will finally be the end of their relationship (fine by me!! Lol). Our only option against him, will be to move in with either parent after one passes. I'm not interested in that option either. I'm afraid BIL will get his way as he usually does by just doing whatever he wants and everyone just letting him to avoid a literal fight.... It's the story of his (and DHs life). I just hope me and my boys will get what we are due.

I agree with this as well. Asking about the process of it all to be able to carry out her wishes effectively is completely appropriate and to her advantage. But asking about the content may be crossing the line depending on the relationship you have with her. 

This is what I would do too.  I would come at it from the standpoint of what you and your DH is doing to make sure that your wishes are carried out in the event of something happening.  I would not ask her specifically what is in the will.  That is her business.  But, if you DH is her executor, he needs to be told that and he needs to be told where to find the original will.  

Thankfully, my mother is VERY organized.  She has a binder where she keeps all of her important information including names and phone numbers of who to call, the name and phone number of her church and the priest's name, the name and phone number of the funeral home for cremation, the name and phone number of how to contact Arlington Cemetery to arrange burial with her husband who is already buried there, etc.  She took me to her bank to get my name on her safe deposit box and I have a key.  She added my name to her bank accounts so that I can access them if I ever need to.  On and on.  I really appreciate how prepared she is. 

You can always open the conversation by saying that you recently got your affairs in order, and what a relief to have that done. Or, you can mention the (potentially fictional if need be) experience of a friend's family when things were NOT well arranged. You can start by asking about medical/end of life preferences to be sure you know what their wishes are (even if there's no substantial estate, it's important to know!). I have to agree that it's none of your business how the wills are written, though if you suspect something unpleasant, you're free to mention it (in the context of "a friend" as well if you want). For example, my mom & her brother were left several pieces of farm property *together* when it would have been much better if each had been given individual pieces. I remind my mom what a pain that business is regularly in hopes she won't do the same to me and my brother! I believe I'm listed as executor for my parents, and I suspect it's a rather boring "half to each" sort of will, but I don't know for sure, and I wouldn't ask.