I’m growing these inside. I let them get sunlight 6-7hrs a day through a window. At most I wait every other day to water them, or when I see that the top layer of soil dry. They received plenty of ventilation

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Pines are not houseplants and they need to be outside. And 6-7 hours of 'sunlight' through a widow is insufficient. Direct outdoor sunlight is 10-12,000 foot-candles - sunlight received through a window is only about 4-5,000 foot-candles at most, about 1/3 to 1/2 of what is required for proper growth.

And watering every other day is likely excessive as well. I'd also question the quality of your potting mix.

I followed the instructions the small book it came with. I didn’t realize they needed more sunlight. Thanks

If you move them outside, which I'd strongly suggest, acclimate them to the sun gradually.

Will do! I’m not sure if moving/separating them to a different pot will help at all but I did so. + I already took them outside for a little. Thanks to both of you for the help! Any further advice is also appreciated

This tree is native to very rocky soil with harsh conditions. I grow it outside, but had to kill a few before I realized that I had to plant it in a mound or up on top of a group of boulders in soil that is 1/2 amended loam, 1/2 lava pebbles. Once established it needs very little water.

Your trees are falling victim to one of the damping off fungal infections. I'm not sure what type on medium you're growing in, but if it's coir, there's a problem. You need a very well aerated medium, and for the plant's sake, you should have the plants outdoors. If I was starting bristlecones, I'd have them in a mix like this:

with a 1/4" covering of screened peat. I would mist the soil gently whenever the color of the peat changed, indicating it was dry. Generally, pine seedlings are root-pruned when very young and repotted as you would a cutting, which produces much better root systems for bonsai.

Al