Have you seen the deed?  Does it say, "H and W, as joint tenants with right of survivorship as to a 50% interest; and S as tenant in common with them as to a 50% interest," or something like that?  Or does it say all three of them are joint tenants?  Maybe there was a side agreement on how the income and expenses would be split.  If you ask Mom what happens when she dies, does she tell you "it goes into my estate and has to be probated, my son is not a joint tenant with right of survivorship"?  Or more likely, does she admit "I don't know."