tilda, people like to throw words like "disgusting" and (my favourite) "vile" around all the time on MN, to describe the mildest of behaviour.
You sound perfectly reasonable to me. Maybe a little bewildered or hurt that your mil hasn't invited you to use one of her homes for a holiday. I get why you are a bit puzzled by it. It's fine to feel that way. You haven't said you're going to confront her, or cut her out of your lives, or take any extreme action at all. There's nothing disgusting about feeling a bit hurt or confused.
Maybe the relationship between your mil and your DH is a bit different to what you're used to. To some people on this thread, it's strange to be confused by her not inviting you. To others, like me, her choice not to invite you is a bit strange. No, she's not obliged to invite you and you're not entitled to it. It's just that I can't imagine any family I know, (including those who own multiple properties), not inviting their dcs to use their holiday homes. If they misused the property in any way, then most parents I know would be able to have words with their dc. Different to say, an elderly aunt who doesn't flush the loo before she leaves. Easier in that case just not to let people stay.
Maybe there is some secret reason for it though (they're renting it out and for some reason don't want people to know that, someone else is staying in it, they have some sort of sex dungeon set up - who knows)?
I'm sure you'll get used to the family dynamic the longer you stay married to her DS.