do not assume your mother even understands herself in the logical sense what is going on, often a woman will only knows she feels a certain way and these feelings are extremely powerful to the woman
Yes, because women are irrational creatures driven by their emotions. Ugh. That is 18th century thinking, at best.
ALL people tend to respond to feelings before thinking, if the feeling is strong and basic enough. Your mom's behavior sounds passive-aggressive to a "T."
Do not confront her; help her access the thoughts behind the feelings. Feelings come from thoughts, but she may or may not be aware of the thoughts she's having. Rather than "What's wrong with you" in a negative tone, sit her down and let her know that you can tell she is unhappy. She may deny it, but then just explain that her behavior suggests she is unhappy, and one or two examples (you use sarcasm a lot more, mom; you roll your eyes now and you almost never did that,--what ever you think is a noticeable change, and just one or two examples). Let her know that no matter what she is thinking, she needs to work it out--talking with you or a counselor.
I suspect there is a lot in your parents' relationship that you do not see or understand. The fact that your dad has "no one" is a huge red flag. That means your mom has been his sole emotional support for years and years and it is very possible he has never returned the favor at an emotional level. She may think--rightly or wrongly--that she must always be the "strong" one. There may be sexual issues you can't help with. Too weird for everyone.
Do not try to replace your mom as your dad's emotional buttress. Offer support, but let him know he needs to develop other resources for support, too--friends, in particular. It is never too late to make friends.
Be sure to sit down with your parents together--if they do not initiate this--and at that point you simply give them feedback about what you see as your position, and how you feel. Let them know that their individual happiness is important to you, and you will not "choose sides."