Marrying your in law?

They aren't blood related so it's not an issue. Related by marriage does not prohibit people from marrying (only blood does...and even then, most cousins can legally marry).

Unless the half sister shares a parent with Declan there's no reason they can't marry. My Mum had two brothers who married two girls of the same family. One brother met and married his wife, then the younger brother married his wife's younger sister. All good.

Is one of Charlotte's parents also the parent to Jack and Declan?

Don't see why not.

It's legal. It's even fairly common. Awkward when one of the couples divorce though.

Yes, perfectly legal.

yes, not a blood relation....

Yup

Yea, that's totally fine

Yes, it is

Did you ask this recently? I feel like I've answered it this week.

Yes, it's perfectly legal.

Still asking this even though you haven't figured out that Declan and Charlotte or not in-laws?

Yes because they aren't blood relatives.

Yes because charlotte and Declan aren't related by blood

yes.

As long as they are not related by blood, marriage is fine.

Totally. Haven't you ever heard of twins marrying twins?