Exhusbands Mother Died Without Will All Three Names Are On Ira And 401k Can One Withhold From The Other ?

my ex-husband’s mother recently died. she had no will. he told me that him and his two sisters are on the 401k and the ira. i want to know if they can not do anything with it or are they required to cash or transfer it? they already cleaned her home out and plan on selling it, are they required to split the proceeds three ways or can they just keep it just to the two of them. they also kept the funeral cards and did not let him even view them, what rights does he have with these two monsters?