Hi Munka22
Welcome back and congrats on the upcoming baby

Your mother in many ways hasn't really been a mother to you and your sister. That is very sad and something many of our members can relate to. Given your past experiences with her, it makes sense to me that you would worry about her returning.
Whether she returns or not, this (unfortunately) is not something you can control. If she were to return, I think it will be crucial for you to work on setting and enforcing/defending boundaries and asserting yourself. We have some material here that can help you with that:
Setting Boundaries and Setting LimitsD.E.A.R.M.A.N.: Describe, Express, Assert, Reinforce, Stay Mindful, Appear Confident, NegotiateThe D.E.A.R.M.A.N. technique is specifically aimed at asserting ourselves and expressing needs for change.
To help you deal with your anxiety, it might also be helpful to look into mindfulness/meditation. This can help you keep your mind focused on the present and less on what might or might not happen in the future. Have you perhaps already tried mindfulness/meditation before?