Sadly some family does go above and beyond when expecting others to help them. This is one of the hardest things most people deal with when they have family. The good news is there are things you can do to help keep relationships good and hard feelings down.
The first step is to talk to your partner. Let your partner know that this has gotten out of hand and based on your needs in the future something has got to give. Ask your partner for support in this matter.
When family asks be honest, let the person know that you are low on funds and what little bit you have you need for school. Be firm, if they ask again simply say no. Do not offer up any other excuses as this will only give them grounds to try and get past the excuse.
You can try to come up with a solid proof excuse if that makes it easier. Some examples are below.
I just send what money I have to a program that secures my funding for school.
I have set up a payment plan for school and between that and my bills I do not have extra money.
I had a old medical bill pop up that I did not know about and I sent my savings to pay it and am now making payments so I do not have any extra money...in fact I need some.
Sorry, right now I have some bills that need immediate attention and I can not spare anything right now.
The key is the first few times........more often then not once you start saying no they will stop asking. They have gotten into the habit of asking you and since you never say no they no longer feel bad about asking. Once you start saying no they will get out of the habit. The fact is everyone is always in need of money.......but if someone is always taking care of their needs they have no reason to not change their lifestyles and make a budget. If you start telling them no they will be forced to take a look at their own spending and make the changes needed to support themselves.
The fact is you have to take care of your own self before others.......not out of greediness but if you can not help yourself how can you help others such as your family or children someday. No matter how you approach this it is not going to be easy......but someday you are going to have to say no regardless so doing so now is your best bet. Once you get in school you will have to say no regardless. So you might as well start now and not risk your own future.