Heather, see if you can view your emotional reaction to the surface-level circumstances as like a 'symptom' of a deeper, underlying 'cause'... Once you become sufficiently aware of the underlying 'cause', you will be able to address and resolve the emotional disturbance that you are experiencing. Please note that successfully resolving this does not entail you having to manipulate, change, or exercise control over the surface level circumstances that led to this 'issue' surfacing.

I'll pose some questions for you which may or may not contribute to you digging a bit deeper to get to the heart of the matter...

-Does your boyfriend's family having this ongoing relationship & involvement with his Ex and her family feel threatening to you because you feel it may contribute to setting the stage for his Ex to re-enter his life and to rekindle their former relationship?

-Do you feel like the Ex-GF having an ongoing relationship with his family somehow signifies that they won't open up to you on a certain level or won't make 'space' for you? Do you feel any degree of exclusion or non-acceptance due to these circumstances?

-Do you have any earlier life experiences that caused you to experience the emotion of jealousy, of not being accepted, or not being being thought of as 'good enough' by certain others? Experiences which may have left a strong emotional 'impression' upon you which has not been fully resolved & healed? Along this line of thought, could your perception of and subsequent reaction to these current circumstances largely be the result of being influenced by stored emotional energy stemming from your past (earlier life events/experiences)? In this context, the current circumstances would be having the effect of stimulating something which runs deeper within you and which requires your acknowledgement and some additional 'inner work'... Try to really connect with how you find yourself feeling and think back through your history and see if this feeling resembles anything you've ever experienced before.

I don't know the answer to these questions of course - but I feel these would potentially be important to ask oneself if one were facing the situation that you describe experiencing. Thanks in advance for considering my questions and for allowing me to comment here.