OP, this is a crappy situation, and I feel for you.
From experience, it will not change.
Do you have dc with your DH?
My STBXH's family are similar. H has 2 dc from his first marriage. I was not OW, or a crossover in any way (they split 5+ years before I met H). His family, from the outset, with no need or provocation on my part, declared that H's ex was their 'real' SIL/DIL/whatever, and I was not.
Yes, they had known her a long time, and were friendly with her. All well and good. There was no attempt at drawing lines from me. They were not prepared to even try to get to know me, as it might upset exW. When H and I had children, both my BIL stated that this was unfair on H's children, and inappropriate and unacceptable, and that their 'real' nieces/nephews would be H's first 2 dc, and not any further dc. They do not consider my (as in H and my) dc as 'equal' relations to them/their dc.
They have consistently invited exW instead of me to various family functions over the years. It is utterly bonkers, and I am not the one who is not being a grown up about it. None of them gave me a chance at all.
H and I have been together for over 16 years (FWIW, longer than he was with his exW), have 3 dc (none of H's relatives have even met our youngest) and it is, quite frankly a ridiculous situation of their making.