1. The merits of giving depend on “to whom you give” too. Thus, giving to bhikkhus in much more meritorious than giving to dogs. But, of course, that does not mean “not give to a specific person/animal” either. You should give as the occasions present.
2. You asked: “So how can we ask “it” to accept the merits?”
You make that determination while giving AND after giving. What matters is your mindset (in this case, your mother’s).
– See, “Transfer of Merits (Pattidāna) – How Does it Happen?”
3. You wrote, “This all happened after my parents’ marriage.”
That is interesting. I wonder whether this “being” could be a diseased old boyfriend of your mother. I have absolutely no idea. But if something did happen to such a person it COULD be the cause.
4. While I can give some ideas, you should seek professional help. I know that you said you have.
– But have you taken her to a good hypnotist? A good hypnotist may be able to get her to recall a past incident (such as #3) that is triggering this.
5. Of course, following the path of the Buddha is the ultimate solution. No other being will be able to bother a mind that is free of lobha, dosa, and moha.
– Such effects just gradually increase as one cultivates the path.
– You may also want to read the section on “Buddhist Chanting”
– Chanting is a common practice in many Asian Buddhist cultures based on the belief that it can keep “undesired beings” away. But you may want to do that gradually. Sometimes the “intruder” may become agitated and may take drastic actions.