These are settled issues. Turns out last remaining parents with multiple children die all the time leaving more than one person to make decisions and settle inheritance.

Spousal rights < parental rights. Step parents are generally (BIG emphasis on generally here, you can what if this shit to death, your specific cases where the parents aren't fit are not what we're talking about here) made to defer to the child's actual parents when making decisions for that child. I can imagine family court, when it comes to that, will come to the same conclusion with polyspousal relationships. The good news is a system exists to deal with it.

Inheritance, like anything without a will, is split equally among the remaining next of kin, in this case, multiple remaining spouses.

It's already advisable to have a Will in place and to discuss what your preferences are medically with your partner, that wouldn't change with multiple partners. Just like a single parent with multiple children.

Even if these issues weren't settled, even if you have some stupid specific edge case that invalidates everything I've said so far, it doesn't fucking matter. These are not problems. They're inconveniences. They can be settled and are no reason to stand in the way of consenting adults seeking happiness at