I am sorry to hear that. By your update it sounds like your dad found the power plays stressful as well, and handled in his own way.
For your emotional well being, while it is important to respect your mother - it is also important to come away from family gatherings feeling good. That is not happening while you feel this way. Don't let it build up until you explode, because you just cannot take it anymore, face up to it like you would another person you'd meet up in everyday life.
If you feel put out by a request, just reply 'no thank you' and move on. If your mother asks you if you want a drink just decline. After every contact before leaving affirm your feelings to your mother, "Bye mum, I love you' and then leave. If your mum starts to act up in regard to being challenged, just state 'don't be silly' and move the conversation on. Don't get drawn into a power play. Leave if she becomes to worked up, and every time on each next visit should she do the same.