I have a brother (not the Commodore) who has, at one time or another, hit on all four of his sisters-in-law. So far as I know, he's not been successful with any of them. Come to think of it, he's probably hit on most of the ladies he's met. He'd probably be mortified if he knew they've all talked (and had a good laugh) about his all-too-transparent attempts. Now, mind you, all five of the sisters-in-law, including his wife are pretty hot ladies, so I guess you can't fault his tastes. I don't think he ever hit on our sister's husband. Give him credit for that.
If all are in agreement, no family problems are likely, and all think they can handle interacting with each other at family reunions, I see no problems. As others have said, it's not illegal, and since the bible says you must, well...
Mr. Alura