Are you kidding me?
No. I'm not kidding.
She left her husband and now everybody around her needs to pay for her trips??
According to you her family (including her ex-husband) has been supporting her financially for years. What she spends this money on is ultimately her business. She could spend it on the tourist trade in foreign countries with sunny beaches; she could give it all away to the poor; or she could even take the money and start a bonfire with it. She obviously doesn't keep it a secret how she spends it though and the only one complaining about it is you.
And since when do people have children so that they can pay the "particular standart of life" of their parent when they are not able to spend their mone wisley?
This has been going on with humans around the world since the dawn of time. This will come as a shock to you but there was a time when "the state" never existed and the only way humans survived was by having offspring so we could continue to look after each other. In many parts of the world this is still the case. Whether there is a state to provide adequately or none at all many children when they become adults still do what they can to provide for their parents and it brings them joy to see their mother and/or father enjoying life after the many years of sacrifices they made while parenting.
Do we as children sign a contract at birth that we are obligated to pay for holidays?
Of course not. Most family bonds are not sealed with a written contract. Family bonds are developed over decades and then it becomes something simply understood and felt. For many this comes about instinctively. For others it is expected and needs to be taught (sometimes repeatedly). And for some like you it is resented either way. Again, what a parent does with material wealth given back to them by their children is a private family matter.
Just the way milions of people on this planet do!
So you feel she is robbing you in some way. Well, being that you are from Spain (a predominately Roman Catholic country) you should be quite familiar with the teachings Rabbi Jesus gave to his followers. Here is one of them:"...whoever takes away your coat, do not withhold your shirt from him either. Give to everyone who asks of you, and whoever takes away what is yours, do not demand it back...."
If one day I have children and the do well for themselfs I'm happy for them and if they find a person that they want to have a future with even better. I can look after myself financially!
If you say you can look after yourself financially then I don't see why you are complaining about your husband taking care of his own mother. If you ever have children I don't see you refusing any money they may give you (or more likely that you will unabashedly ask them for) especially if it means you can treat yourself to an exotic trip to someplace you always wanted to go but couldn't because of your "evil" mother-in-law.
And if after so many years of working she hasn't got any money aside then it's just sad.
But that's not the account you have given us. You said she has money coming in from all directions.