Why would she need to keep any monies separate in this event? Her estate simply goes to her chosen heirs. She could phrase her will this way if she wishes (approximate phrasing, just one example):
"First, if I should predecease Mr. X, I leave my entire estate to Mr. X.
"Second, if Mr. X should predecease me, I bequeath a sum equal to what I inherited from Mr. X to his surviving siblings, divided among them in equal parts.
"Third, I bequeath any remainder of my estate to my nephew Mr. Y."
But I don't understand why Mr. X wouldn't just make sure his wife is very well taken care of financially (life annuity plus lump sum) and then just leave the remainder to his siblings directly — and/or give it to them now. Why impose this burden on her, that she must bequeath anything to his siblings? And that they must wait for her to die before they get to enjoy anything money can buy? They could easily predecease her anyway!
The key to remember is that Mr X and Mrs X are childless and both have their own siblings - plse refer Scenario that I posted in post #1. Perhaps yr outlook might be different but just as Mr X does not expect any bequests from any of Mrs X's siblings, neither does he feel obliged to leave bequests to any of Mrs X's siblings. Bearing in mind too, that Mrs X has assets of her own and Mr X fully expects that some if not the majority of these assets will be bequeathed to Mrs X's siblings in her Will
Mr X loves his wife and does not mind if Mrs X uses all that he possesses while she lives but he would want any unused assets to revert to his own siblings. Without a crystal ball, Mr X does not know how long Mrs X will live after he is gone. To ensure she has sufficient funds to cover any contingency, let's say Mr X bequeaths his wife a lump sum of $2mm. But due to unforseen circumstances, let’s imagine Mrs X passes on just 1 month after Mr X. In such circumstances, if there had been no prior discussions, the $2mm would be distributed according to Mrs X's Will and presumably to her siblings. This is the situation that Mr X wishes to avoid
On yr questions:
1. Why the need for Mrs X to keep monies separate if there is no Jt Ac? Answer is that Mrs X has her own assets (incl monies)
2. Why must Mr X's siblings wait b4 they get to enjoy...". This can be taken care of by Mr X also leaving some lump sums to his siblings directly as per my earlier post. But Mr X's first priority is still to his spouse and that is reflected by the Jt Ac arrangement and the life annuity with Mr X's sibling as nominee
BTW this phrasing - "Second, if Mr. X should predecease me, I bequeath a sum equal to what I inherited from Mr. X to his surviving siblings, divided among them in equal parts." is not entirely correct. Mrs X needs to only bequeath the Balance of the sum that she inherited from Mr X. Without a Jt Ac or proper accounting, it would be impossible for an executor to determine the Balance