Need seat recommendations

I've been trying to research everything and I'm just going to go nuts!

I need two seats. One for my 3 month old and one for my 2.5 year old. I've settled on a marathon for when 3 month old DD2 outgrows her snugride. I also need a seat for DD1. I was dumb enough to register for the Alpha Omega Elite and she'll outgrow the 2nd highest harness position very soon.

We drive a 2000 Olds Alero 4 door sedan. No latch. DD2 is long and thin. She is 2.5 years old, 38" and I really don't know what she weighs. She is on the slim side though. She is fairly long waisted.

Mostly, I'm looking at the Radian and the Frontier for DD2. Frontier is FAA approved right? It is important that we can fly with whatever we have. What is the expiration time line for those two seats? I'd like it if DD 2 could maybe get a bit more use out of DD1's seat (frontier/radian) once she outgrows her MA. Can we get a good install without latch with the radian? Do I need to have latch tethers installed in the car if we go with a Frontier? I've been trying to read up, but really I need someone to walk me through this a bit.

and on the more frivolous side... for whatever you recommend for DD1 what marathon cover would compliment it? In buying for two sisters I want to make sure they are "equal"

Have you looked at the Graco Nautilus? It is a forward facing seat with a 65lbs. harness, tall harness slots, easy to install, FAA approved and turns into a great booster seat (that you can latch!).

I also was thinking about the Graco Nautilus for your eldest. But, to spread out the costs, I was thinking of getting the Marathon first. Have your DD1 use it now. Then when the baby does outgrow her snug ride, buy the nautilus, move the baby to the Marathon, and DD1 to the Nautilus. This way you could spread the cost out, and stretch your time by 6 months or so to get a newer DOM on the Nautilus. The Marathon will likely be outgrown by both your children by the time the 6 year expiration is up. But you benefit from getting some extra time on the Nautilus. 6 years for Harness and HBB mode, 9 years for LBB mode.

Also, if your 2 1/2 year old is under 33 or 35 pounds (depending on the DOM of the Marathon you get), then it is possible that you could turn her back rearfacing in the Marathon for a bit longer.

Thanks for the response! I had considered the nautilus, but having had a crumby experience with the Alpha Omega and its horrible twisty straps etc... I was feeling pretty ready to upgrade to a "nicer" seat. However, money is an issue and if the Nautilus is just as good that is what we will do.

I had actually already been thinking that I should get the MA first and "loan" it to DD1 for awhile. However, before I buy the MA I'd like to have an idea of what I'm getting for DD1 so I can buy an MA cover that is "equal." I don't care about it being matchy... but if I end up getting the princess radian I want a girly MA cover. if I'm getting a nautilus I'll choose onyx. I'm not sure what MA cover would go with the pink frontier. Anyhow, if I go for a mariposa MA, stick my uber girly 2.5 yo in it and then end up needing to switch her into an ugly nautilus so baby sister can have the pretty butterfly seat it will NOT go over well at all. LOL

Any install issues with Nautilus/Radian/Frontier in a 2000 Olds Alero 4 door? Remember, we don't have LATCH. Any caveats to any of those seats?

What DOM has the 35lb RF limit for the MA?

I feel like I made a really poor choice with the Alpha Omega Elite. I liked that it could RF to 35 lbs and had NO IDEA the harness height would top out on me so soon. I thought I researched it well before I registered for it. I didn't read anywhere that lots outgrow it by/around three. Anyhow, because of that I'm really really concerned with making good seat purchases this time that will last as long as possible. So thanks in advance for answering all my questions.