can a heir in a will lose their right to inherit if found looting from the estate

It would be possible, yes.  you would have to file an action against the estate, and claim that your sister took the money from your mother, and prove how she did it.  for example, if your mother had given your sister a power of attorney and your sister misused it, or spent the money on herself instead of your mother.  However, if you sister is the main beneficiary of your mother's estate,and you provide she took money from your mother, is it going to affect your share of the estate?  this type of litigation can take a long time and can be costly, but in most circumstances, the estate has to pay the legal fees.  You should discuss this with a lawyer to see if it makes sense for you to pursue your claim.  Good luck.