Your problem isn't your relatives. Behaving as though people can be trusted or taken at their word is "avoidance". You know when people are not trustworthy (relatives or not). As long as you act like they are you will be used and abused.
"Nice" is not a positive. "Nice" placates. "Nice" lives in its own immature bubble.
As our friends in the WTBTS put it: "Awake!" We're not living in Paradise. You're going to have to defend yourself (not just from relatives), so study up on topics like "manipulation" and "masochism". There's where you'll learn what to say and how to come across as someone who can't be walked all over.
Do it now, before you have the baby. Then you can model appropriate behavior as your child grows. That can only benefit both of you.
Best wishes,
AB