I started a thread on much the same topic about 6 weeks ago
here.
I was slightly surprised at the results: people seem to be very conservative about this. The most common replies seemed to suggest either extreme pessimism ("My folks might well need 20 years of nursing home care and burn through the whole pile") or a feeling that the whole matter was a little distasteful.
While I certainly wouldn't base my entire portfolio on a $500K lump sump appearing before my 60th birthday, I think that for some people, it can be legitimate to figure in different amounts with different percentages of likelihood. After all, that's what we do when we run FireCalc on any portfolio, even if we go for the "show me an SWR which would have succeeded 100% of the time" option - the sample is "only" 120 years or so. I don't see an objective reason to exclude inheritances, while at the same time assuming, for example, that stocks will return to their historical growth rates before the end of the next decade. (I believe that they will, but if I'd been retiring with a portfolio of Japanese stocks in 1990, I'd have been badly burned.)
It's also true that inheritance was never a taboo subject when I was growing up. While I didn't know my parent's net worth, I knew where all the certificates were. My Dad handled the execution of the wills of several relatives and showed me some of what it involved at the time.
DW and I can reasonably expect to inherit half each of what my mother, and her parents, leave. We can also place a reasonable present value on that in each case, because for my mother I've seen the numbers - it's one of the easier subjects of conversation when I visit - and for the in-laws, BiL has PoA and his first job was to check that their assets meet their spending requirements. So we can make assumptions based on a reasonably likely spending-down of those assets: X% chance that we will inherit 70% of PiLs' current net worth, etc.
For example, MiL is bedridden and "not all there" following multiple strokes, but built like an ox. FiL is 89 and caring for her, with cardiac issues and regular visits to have melanomas removed from his skin. We assume that he will die first and that she will need some years of nursing home care. One option in that case would be to realise some assets (probably selling the house) and purchase an "immediate needs annuity", which would guarantee her care for life while protecting the rest of the capital. It might cost half the value of the house. I don't think that that's distasteful; I would call it prudent.