My sister in-law is a widow. Her husband died about 5 years ago and left her well off with rental property and a very large amount of life insurance to take care of her. She went on a shopping spree and sent all her money and then sold her rent property for half of what is was worth and spent that also. After being broke, she moved in with one of her older children who was doing drugs at the time and had to move out. So my wife took out a loan for a used RV and had it placed on our property. Her sister at first paid for water and electric at a reduced rate because she could not pay all of it. She had a job at the time, but would spend her funds on other things. Then she decided to stop working and get on SS disability for a so called back problem and mental disorder. We had to pay all of her expenses for six months till she was able to get her disability from the government. Now she can't afford to pay anything to us at all. We pay the loan on the RV and all of her water and electric bills. My wife gives her gas money most of the time for her car. She is what I call a road runner. I have told her she needs to get a cash job on the side to help out, but she said this is out of the question. So I am stuck with her because my wife wants her living with us. My wife will retire in two more years and I have thought about it, but not sure what to do because of my sister in-law. How about some help here people.

This is between you and your wife. The details of the sister's past aren't really relevant (except I imagine it adds to resentment).
Sit down with her and go over the expenses. In fact go over all your expenses, and a retirement plan. Get help from a retirement planner, in person if you can - some places like AARP do that. This will probably result in some compromises about how much you spend on her sister. If she is on SSDI, she can work up to a certain amount each month (real wages, not cash) and not lose it. I see no reason why she can't be given 2 weeks to start looking and 3 months to find a part time job, and pay her own car expenses and RV utilities, and eventually the loan too. Either that or she can get on the waiting list for disabled housing (same as senior housing).

Your question title, 'My sister in- law is a bum', really says a lot about you.

You refer to her "so-called back injury and mental illness" in a way that suggests she's faking, and scamming the government for financial assistance.

She has gone from, what sounds like, riches to rags, and something has triggered her downward spiral.

If you were concerned more about her, than yourself, you might try the following.

1. Have her health checked out, by a doctor, preferably a Psychiatrist. Find out what exactly what her mental and physical health is. You pushing, and expecting her to find a paying job, and pay for her expenses, may be beyond her control, and/or her ability.

2. Depending upon what diagnosis is made, or confirmed, you may need to seek guardianship of her, and her finances, in order to make sure her basic needs are covered.

3. See what options are available in that regard to apply for additional assistance, financially, to help pay for the trailer- the insurance, upkeep, etc.

4. Legal help might be advisable to see what you can and cannot do, if she is in fact, not disabled, as you have suggested.

5. Do a little research on her disabilities, and find out if there is something concurrent going on such as alcohol abuse, gambling, etc.

If she cannot control her money, and she is living a life that is out of control, find answers, and quit complaining about what a hardship this is on you. I'm sure it is equally difficult for your wife to understand what is going on, and to also be between you and her sister.

Until you take some basic steps to find the answers you need to make this situation better for all concerned, you adding expectations to an already dismal situation isn't going to help.