Calling stepmoms/divorced moms please help this is all new for me whats my role as stepmom once married?

been dating this great guy for over 1 yr now who has joint custody of a 7 yr old.I had limited contact with his son because i was just "the gf" but now we are engaged and moving in with each other.His son lives with his exwife but he has him 50% of the time.Both his parents are very involved with him and his mom is very possesive of him. I don't want to make trouble with his ex(she allready dislikes me and marring "her ex" has not helped things) so can you tell me some dos and don't for new stepmoms?I don't want to overstep my boundraires with her regarding thier son.