WarwickAlice
YANBU to be upset that your SIL has accidentally 'stolen your thunder'.
YANBU to be angry that your in laws treat your dh badly in relation to his sibling.
YABU to blame your SIL.
Talk honestly to your husband, ask him how he feels before spilling the beans on how you feel. Ask him how he would like to handle this.
In your shoes I would go into this new relationship with PILS being GPs with an open mind. Maybe this new baby will be loved by them and they will do their best to be fab GPs.
Great.
Your child will have a cousin the same age and probably in the same school year (almost certainly). Quite nice for them (hopefully).
IF your PILS prove true to form and favour the other child you and your dh can decide on how to proceed.
In my book it would go:
Initial conversation with PILS to see if they are aware they are doing!
More conversations with PILS to stop them doing it!
Withdraw from PILS and minimal contact.
Children do not need negative forces in their lives, and neither do adults.
You do not need to go no contact (unless you dh wishes to) but you can go low contact.
You can see plenty of SIL and her kids, if her behavior towards you is fine.
Stop crying, regain composure, your baby has you and your dh, he or she will be loved and cared for. Your PILS are sad people who may well miss out.
But be prepared, they may surprise you.