Blended families can be challenging to navigate, and in your case, your father married the mother of a classmate you had strong negative feelings toward back in high school, adding to the difficulty. But if you truly want your children to have a relationship with their grandfather, you’re going to have to look more closely at yourself, which will require you to separate the past from the present.
Although you’re an adult in your 30s with a family of your own, you present this dilemma from the perspective of what sounds like your younger self. You may have had very valid reasons for distancing yourself from your parents during your teen years, and your parents may not have earned back your respect. But as much as your adult self sees the value in fostering a relationship between your children and their grandparents, your lingering childhood feelings toward your father seem to be getting in the way here—much more so than who his wife happens to be.
To tease this out, let’s consider your grievances with your father’s wife. According to your letter, you find her obnoxious. I don’t hear that she’s willfully cruel, manipulative, or dishonest. You don’t say that she has dubious motives, such as spending money that might be rightfully yours. You don’t mention any major issues that would require professional help, such as an addiction or a serious mental-health condition. She seems to make your father happy. In other words, she sounds unpleasant but harmless.
Most people would be annoyed by an obnoxious new addition to the family, but annoyance isn’t usually grounds for estrangement. Something else seems to be going on here, and you articulated it in your letter: When it comes to your father, you’re looking to “have an excuse to finally leave my relationship with him behind.” In other words, you seem to be using this battle over your father’s wife to work out something between you and him.
Instead of focusing on what you don’t like about your father’s wife and then ostracizing her, you and your children will benefit if you work through your anger toward your father (perhaps with the help of a therapist). If you do so, you’ll learn to relate to him as the adult you are now, not as a version of yourself from the past. This will help you view the situation through a much wider lens.
For example, you’ll be able to see that what you characterize as your father’s wife’s “meltdown” might actually be a very normal human reaction to repeated and profound rejection for, to her mind, no ostensible reason. My guess is that she feels like she’s made an effort to be nice to you, but being nice to somebody who’s overtly hostile is hard. Which is to say, your father isn’t the only one with a temper; yours just presents differently. You might also see that you’re punishing your father for his past parenting mistakes by putting him in an untenable bind when you make him choose between spending time with his son’s family, at the cost of causing his wife great pain, and being a loyal partner to his wife, at the cost of a major rupture with his son.