Congratulations on your two tiels. It sounds like you have your work cut out for you. I agree that they need a bigger cage as soon as possible. They might end up fighting in this small of a cage. You might have to slowly get them used to being in a cage. Give them their favorite treats when they are in their cage, and keep all their food in their cage. Save their favorite treat just for training m. Often time tiels favorite is millet. I also wouldn’t use nest boxes. If you want your cockatiels to be friends with you, you don’t want to encourage breeding, like I saw you were thinking of in another post. Breeding birds often don’t make good companions. Getting dummy eggs is a good idea. Having some sort of a routine with them will help them settle in. Also, reading to them, dropping treats into their cage, and never forcing them to do anything will help. Having a play set in your room for them to hang out on might be a good idea. I would discourage them from finding any other nest sites in your room. After several weeks when they are more comfortable in their new home, you can try training them to step up on a perch or target training. You can look up how to target train on the internet. Look up Barbara Heidenreich training videos. You need to gain their trust, which is a slow process, but if you are patient you will get there. Just remember to reward good behavior, ignore undesirable behavior, never force them, respect their body language, avoid getting bit, and go at their pace. They often hiss when they are scared. It’s not that they don’t like people when they hiss, it’s just that they are scared. Also, I’m sorry about your mom, and I’m sure they are just scared of her.
Also I almost forgot, as for diet, feed them what they were eating before you got them, but eventually you should change their diet to include seeds, pellets, and fresh veggies.