Third-wheeling is awkward, but it's especially awkward if you feel like a third wheel with your husband and his ex-wife. Every year, a woman we'll call OP and her husband would travel. This year, OP was thrilled that her husband's son wanted to join them. She had been a steady presence in the kid's life for years, but his mom, her husband's ex, liked to one-up her. This ex told OP that she needed to join them on their travels as well, as there was no way that the kid was going to be allowed to…