For context: I have anaphylaxis allergies to milk, seafood (all fish, all shellfish, etc), peanuts and tree nuts. The seafood allergy is not just ingestion, but also airborne. The peanut allergy has in the past been airborne when I was exposed to a massive amount of crushed peanut & peanut shells and their dust, so the potential is there for an airborne reaction to (enough) heated peanut but so far I have not experienced that (OTOH I really don't go to places that would serve that anyway).
For our cruises (we only cruise Disney) no seafood is allowed at our table (which is easy because it is just my sister and I). In addition, we specifically ask for our own table because of the allergy -- IMO it would not be fair to prospective tablemates to prohibit them from having seafood too, but it would be necessary for my safety. Easy solution is our own table and on our two cruises so far that has not been a problem to get. Also,
DCL has been good about situating our tables kind of out of the way and in decent ventilation. Before our first cruise we had an extensive conversation with DCL special services about accommodating the seafood allergy, especially in the MDR -- they were really helpful, understood the issues, and had great suggestions.
My sister is free to order as much milk, peanuts and tree nuts as she wants

We have never had an issue with being able to do that. For bread we choose to just have bread that is safe for me (on our last cruise on the Fantasy they had this most wonderful sourdough rolls that we could not get enough of!!) -- I got a dish with safe earth balance margarine in it, and my sister got her own dish with butter. She gets ice cream for dessert while I get the chef-being-creative safe dessert.
I HAVE reacted to airborne seafood in an MDR and would prefer to avoid that occurrence again.
Finally, for me pre-ordering the next night's dinner each night works really really well. On our next cruise I think we will also pre-order MDR breakfast and lunch, which will make those meals easier too. It means spending a bit longer in the MDR, but we are always late dining so that is not a big deal. Also, it has meant I was able to get some things not on the menu -- e.g. one night there was a great butternut squash soup. For dinner two nights later I asked if it was possible to get that again and they were able to make it for me

Also one night french onion soup was on the menu and they made me safe french onion soup -- something I have not been able to have for a very long time. I find it gives me more menu flexibility because with the ~24 hours notice they are able to make more things (e.g. if something needs to marinade they have time to make a safe marinade).
SW