Muslin holds up to a second child better than most baby fabrics because of how the weave behaves over time. The open weave doesn't wear thin with washing, it actually gets softer, and Epic's clothes are double-layer muslin cotton with flat seams, giving the fabric more structure from the start. That durability is also why how long muslin baby clothes typically hold up stretches well past one child for most families.
What actually survives to a second child usually comes down to how the clothes were washed the first time around. Machine wash cold on a gentle cycle, tumble dry low or hang to dry, and skip the fabric softener and high heat, since all three keep the fibers from breaking down early. We cover the full routine in how to wash muslin clothes so they stay soft, and treating a onesie that way from day one is what makes it ready for a second child later.
Storage in between matters as much as washing. Muslin is cotton, so packing it away damp or sealing it in a box for a year or two can leave it with the yellowing that shows up on stored baby clothes, which is avoidable with the right drying and storage. If that happens anyway, how to get yellow stains out of stored baby clothes walks through fixing it. Dry everything fully, store it somewhere it can breathe, and clothes that worked for your first child should be ready for your second.