As a person raised by different mother figures in my life my advice is,

1. Determine your motherly role but don’t try to be their mother. Mothers can never be replaced. Know the boundaries.
2. Don’t try to discipline then without involving their primary parents
3. Don’t try too hard even for your own sake. Act normal and everything will flow.
4. Congratulations on your new family and I wish you the very best 🥰