Not sure if step sis-in-law has a jealousy issue regarding my pregnancy?

maybe confront her and explain how you feel, if she feels youve taken the limelight shes a bit childish to be having children because its unlikely anyone is going to favour one child over the other. remind her that it will be good for you to both be pregnant together and your children will grow up close. if she sees the positive rather than jealous aspects she might stop acting so cold towards you. if nothing changes then leave her to it, its not your problem to baby her through life, if she wants to sulk let her she is the one who will have a pregnancy she hasnt fully enjoyed out of her own pettiness.