But I'm wondering... how do you know that DIL organizes everything? Did DS/DIL tell you that or are you assuming? Cuz many young couples, today, as I understand it, have a yours/mine policy - as in, "You deal with your relatives and I'll deal with mine." So DIL may work out all plans with her side of the family and expect DS to do so with his. Only, of course, he needs to check with her to make sure she hasn't already made other plans (and, IMO, she should do the same with him). I realize you can't do/say anything about whether or not they clear plans with each other. I'm just just trying to think of all the possible reasons why she might only be including her own relatives. It might be cuz she's just totally selfish and insensitive, of course.
Also, yours/mine policies often are adopted when one or both spouses have somewhat strained relations with their ILs. Since DIL made that awful remark to you about "apron strings," it doesn't seem as if your relationship is all that good, even though it doesn't appear to be at all your fault. So might explain why she leaves all plans with you up to DS. But again, he may need to make those plans further in advance.
TBH, though, it sounds to me as if DIL is the one who needs to cut the apron strings from her parents, etc. But I realize there's nothing you can do about that either. And mentioning it would only backfire on you. Just hoping that, in time, her ties with her FOO (family of origin) will loosen a little - enough so that there is more room for you. More (((hugs)))