I spent July Fourth weekend down the shore (sorry, can’t shake my Philly roots) with my two kids and their significant others. It could have been a disaster. After all, I know, I wrote the book. But it was wonderful.

If you’re the mom, it’s easy to take for granted that your son likes his sister’s boyfriend, and your daughter likes her brother’s girlfriend. But when this happens, take a deep breath, look heavenward, and thank your lucky stars.

When your sons and daughters don’t like their sibling’s girlfriend/boyfriend/wife/husband your family will never be quite the same. Holidays will feel a little more stressful, birthday dinners a little emptier, splitting family assets a little nastier. And frankly, based on the interviews I did for the book, and from the women I meet at book events, the sibling-in-law clash can be much more challenging than the mother/daughter-in-law issue. In fact, while many women have told me they like their mother-in-law or their daughter-in-law, when I throw the sister-in-law into the batter, I get a fair share of, “Oh, her.” Oftentimes followed by, “She’s a bitch.”

So is there something we can do to help facilitate a good relationship between the siblings-in law? Yes. Even the most seemingly impossible and improbable relationships can work. They just may require a grand effort.

If you’re the significant other and his sister ignores you or is just outright rude to you, rather than clam up and act bitchy, engage her. Her arrogance will be forced out into the open for all to see, and by all, I mean your boyfriend and his mom. Invite her to do something with you without the mom. When you’re caught in the mother-daughter mix, you will likely feel like a third wheel, so extract her from her mom. And DO NOT criticize her brother to her even if she eggs you on. Anything negative will be stored in the deepest recesses of her mind. She can criticize him. You can’t. At least not in front of her.

If you’re the sister and you really don’t like this girl, you have no choice but to act supportive of the relationship anyway. If your brother truly seems miserable, it’s okay to say with soft, sisterly concern, “Are you okay? You haven’t seemed yourself?” He may think you’re referring to his job. But if he is having any doubts about his relationship, you’ve given him permission to make a change and you haven’t even mentioned her name!

On the other hand, if you don’t like the other woman but your brother seems happy, learn to figure out what he sees in her. And then spend one-on-one time with her, invite her to go for dinner or to a movie without your brother, and most definitely, without his mother.

It’s harder to get a mother-in-law to come around if her daughter doesn’t like you. Not so much the other way around. So work at first on creating a relationship with your sister-in-law (present or future).

And then I hope you, too, find yourself with happy four next Fourth.