I don’t have any suggestions for spell jars here however @Francisco shared a beautifulPagan blessing for newborn babies on a post I made a few months ago.
Have you considered writing your own spell? Rosemary is a universal herb and can be used in place of any herb that you lack for a spell. I might possibly would include lavender or chamomile (chamomile tea bags are super cheap and work wonderfully in a pinch).
Another point that isn’t quite clear but should be considered in this situation is whether or not your sister will give consent for you to cast on her behalf? If you don’t believe she’ll consent or don’t think you have a lot of times to get consent and you’re already limited on supplies I would suggest maybe making a protection amulet or talisman for mom to hold or wear during labor because it kinda works through loopholes on not having consent. There is a fairly easy cast along for Making amulets on the spells8 site.
With permission I would like to add you to my prayer book and light a candle for your family.
Blessed be 