I’m itching to just ask why?

In this particular case the daughter-in-law is clearly not being treated very well... so there is something odd about it Smileless

Well I don't know if it is odd TBH NotSpaghetti; not very nice I agree but not all s'.i.l. and d's.i.l. have good relationships with their wife's/husband's parents and vice versa.

It must be lovely to be loved like a D by your m.i.l. but as long as you have a reasonable relationship, it's not the end of the world if you're not; at least it shouldn't be.

In her OP lilacs describes their relationship as ^very positive and good^; what a shame that that isn't enough.

I think being expected to be treated Ike a daughter or exactly like my husband would look like if I expected her to call me everyday like she does her son or if I expected to be in her inheritance or something. She calls my husband almost everyday and I certainly don’t expect her to do the same for me or leave her in my inheritance but yes as her son’s partner if she is paying for his vacation with them and leaving me to fend for myself that’s a little odd to not treat us in those particular circumstances as a married unit. My MIL could have given my husband a set amount and said, “hey this is for you and your wife to use towards the trip however you like whether there for airfare or lodging” instead of being like, “here you go son this portion is for your trip but forget your wife/my DIL she is on her own. She could have given the same amount she wanted to financially while including me instead of spending any extra more. There are ways to make the spouse feel included. Or giving him side money and telling him, “oh you work so hard you deserve to treat yourself” and while yes I agree 100% he works hard outside of the time. I have young children at home so even though I work outside the home my work is just as valuable and I bring just as much to the table raising our children full time day and day out saving us money on other expenses and allowing him to go to work while I am raising his children. That is a huge job. So I take that as, “so I don’t work hard and am not deserving of a treat only my husband’s contributions are valued?” Like what!? My MIL is a fellow woman she should understand the struggle of being a mother to young children and besides that women’s work is already undervalued and under appreciated as it is. Trust me men aren’t exactly overlooked in society

Not to mention again we are married so the running of the household is team effort not just due to one’s hard work. So a bit odd to recognize the hard work of one person while completely overlooking and ignoring what the other spouse contributes and brings to the table.

And my husband did bring up the whole vacation and paying issue to his mom and treating him while ignoring his wife and she just got defensive with him saying oh you always defend her. Like duh of course he always defends me we are married I’m his wife!

As someone said upthread maybe she didn't have that sort of relationship with your father-in-law.
Maybe she felt alone?

I'm obviously speculating here...
We don't know what she thinks - and even your husband can't seem to find out!