My sister has been made trustee of my mother's money and house. I just found out she opened up a home equity loan in her name. Help!

Sister is Trustee of what?
what kind of Trust?
is house titled in Trusts name?

if your mom was the settlor of the trust and is the beneficiary of the trust, she still has rights regarding the Trust. If she lives with you, you can take her to the nearest branch of the bank who has her accounts and did the Heloc to meet with a bank officer to get copies of all banking and lending done as she was the settleor and is the beneficiary of the trust. Bank officer will likely want to speak with her one-on-one in their office. So your mom needs to be somewhat competent and cognitive to do this.

If the amount is correct the source of money is not SSI but Social Security. Both programs are administered by the Social Security Administration. The maximum benefit in 2019 for SSI is $771. Different sources of money fund the different programs.
A trustee administers a trust but you didn't explain anything about a trust for your mother. What is that about? Is the home held in a trust for your mother? Do you have a copy of the trust or know what assets it holds?
If your mother's money is paying the mortgage or loan on her house and utilities there there would be less money available to send for her care in your home. What was the plan when your mother came to live with you? Did you discuss any of this ahead of time?

Contact the SSA to report Financial abuse. You will hear from them.

Please, do not drop your mum off at your sister's home. She is not a handbag...

All the best. Money clouds unethical peoples judgment.

Her home can be sold, "As Is." No need to Upgrade for a sale...

Note: If your mum is competent to make decisions, have her give you a Durable Power of Attorney? It is Valid as long as she is alive.

I had to use Moms SS check to pay taxes and upkeep on her house when she lived with me. She received a small pension I used for her personal items.

I would ask your sister why the loan. If she can't show that its going towards the house then she needs to pay Mom back. Do you mean trustee or POA? Whichever, sister may be abusing her authority and you can ask for an accounting.

My mother's home needed 15k worth of upgrades in order to be sold. I'm not talking about a renovation. Plumbing and electric had been done by my dad and friends without pulling the proper permits.

Fortunately, there was money in her accounts to do this work.

Also - sorry this isn't really my area but I have heard tell - have you been in touch with whichever office issues the SSI checks? I gather they take a dim view of these benefits getting siphoned off and may wish to hear what's going on. If you don't have the contact details maybe your mother's bank could provide transfer references and similar information.

This is actually a dispute between your sister and your mother: your mother is the complainant and/or plaintiff in any action to be brought, and you are only your mother's representative.

So: you don't need to afford an attorney, it's your mother who does. And she has her SSI check, and her house. Approach an attorney on that basis, on your mother's behalf, for advice. And report your sister's activities to the police meanwhile.

Why on earth would you house your mother if you don't have access to her money?

Is it possible to take mom to sister's home and drop her off?

Have you gone to Legal Aid, or tried a University Law school clinic?

At the very least, notify the bank in writing that this is a fraudulent transaction on your mother's account, if that is what it is.

Are you certain that the loan is not to bring mother's home up to code before sale?