Abusive housemaid (employed in the family for more than 30 years)

You should consult a probate attorney in your area. You are legally adopted which makes you the legal daughter of your parents. You have the same rights as if you were their biological daughter. You need to find out if the maid has done things to gain control of your parents and their assets. For example, do your parents have wills? If so, they should be reviewed by the probate attorney in order to make sure that the maid hasn't exercised undue influence (for example, having her named in the will as a beneficiary). Has the maid somehow forced your parents to sign a Power of Attorney giving her access and control of their assets, etc.?  You need to check your parents' bank accounts and other assets to make sure that the maid isn't stealing from them.

The maid's actions and hostility towards you appear suspicious and she may be trying to hide what she is doing from you. After consulting with the probate attorney, you should consider terminating her employment with you and getting proper care for your parents through a reputable caregiver service.