I have been very satisfied using the Logitech Crayon for iPad because it works well across every iPad model released after 2018. This solves your compatibility concern between the Air and the base model perfectly since it doesnt require specific pairing. From a technical standpoint, pressure sensitivity is only necessary for high-level digital art and shading. For a bio project involving diagrams and note-taking, you wont notice the difference at all. The active nib technology in the Crayon provides the same palm rejection and tilt support found in more expensive models. If you want to save even more, the Jamjake Stylus Pen for iPad is a solid budget alternative and I have had no complaints using it for basic tasks. It offers about 10 hours of battery life and a similar 1.5mm fine tip. It is a much more logical purchase if you arent planning on doing professional illustration work... just make sure it charges via USB-C so it works with your newer Air charger.

Like someone mentioned, the pressure sensitivity is the main thing you are paying for with the expensive models. I find it a bit stressful trying to decide because compatibility is such a mess between the different iPad generations. If you want to play it safe for both your Air and your sisters older model, you should check out the Adonit Dash 4 Dual Mode Stylus. It has a physical button that switches between a green light for older iPads and a blue light for newer ones. It doesnt have pressure sensitivity, but it is reliable and works on basically everything without needing a Bluetooth pairing. If you think you actually need the pressure for your drawings, the Adonit Note Plus Stylus with Pressure Sensitivity is a solid alternative to the expensive Apple ones. It is much more budget friendly but still offers 2048 levels of pressure and shortcut buttons. The build quality feels way more substantial than those 20 dollar knockoffs you see online, which feels safer if you are carrying it around in a backpack every day. Honestly, pressure sensitivity is mostly for artists doing heavy shading. For a bio project, you just need clean lines and good palm rejection so you can rest your hand on the screen comfortably. The cheap clones usually have poor quality control, so you might get jagged lines or connectivity drops right when you are in a rush. Sticking to a known brand like Adonit is a safer investment imo if you want the stylus to actually last through next year.