I’ll take a wild leap here and assume a lot of people can relate. Even if we’re talking annoyances other than table manners, in-laws are people you didn’t pick out, you probably haven’t spent tons of time with before, and that you now have to love like family. Weird.
Adding to the weird, as family you get a glimpse at all of the behind-closed-doors behavior of a family, but without that happy foundation of years of love. Depending on their dynamic, they’re possibly going to “be themselves,” acting in ways they do when they’re comfortable with family. That’s not always pretty. But, there’s the slight, (hopeful?) chance that they’ll know how to behave in public—for example, at a wedding.
If you don’t believe me, and are still worried that there might be some awkwardness at the wedding, consider introducing your parents beforehand. I know it’s not always possible, depending on geography, but it’s nice to round up all the parents and get them acquainted in advance. In this situation, the bonus is that your mom can meet mom-in-law for herself. If it’s truly a terrible clash, she can sort out for herself how to interact when the wedding comes, and generally brace herself.
More than just the wedding, though, you do have to come to grips with the idea that they’re going to be around for a good long time. You’re signing on to embrace them as family forever-ever. That could sound scary, but frankly, a lot of us have these hurdles with our own immediate family. Folks we love and cherish and wouldn’t trade for the world can also manage to be annoying or brash or loud. It’s just all the easier to notice this stuff in other people, people who aren’t yet close to us. Meaning, it’s quite likely your own parents are annoying and maybe a little gross in their own special ways.
Luckily, you can still grow to love them, which doesn’t erase the annoying bits, but does soften them. If I’m being honest, it doesn’t happen in every situation. Maybe instead, you can work on growing to respect them, and their relationship to your partner, and the things they did right in raising him. For now, you have to just accept them for who they are and be nice. Figure out how to cope with the stuff that really gets to you and maybe mentally brace yourself each time you go to visit. Focus on the positive. Not for their sakes, but for your own. Because there’s no changing them, and they’re sticking around.