my mom is being used and she doesn't even know it

I'm 17. This has been going on for years. Their not even family. (Changed names) The backstory. My moms sister Lynn is married to Mike, mike had a kid (Sam) with  Maria before he met Lynn. Mike was never in Sam life. Mike is my uncle sine he married my aunt Lynn. Some how we lived next to Maria & Sam. My mom accepts them as family. She gives them money if they need it for "sams 1 year old" (she uses it for drugs and cigarettes) they use our car saying they'll give gas money when they get it (they never do) when they use our car they say there going somewhere and than go somewhere else (lying) so they lie and steal. After my mom gets fed up with it she starts saying no and than they back off for a few months and then come crawling back. So my mom says yes, it's a vicious cycle that my mom doesn't understand. I'm trying to get through to her but nothing is working. My mom cries because we are struggling with bills and food but somehow she has money for them. I don't know what the fuck to do. I know what your gonna say or your 17 and she's an adult and blah blah blah. She obviously isn't a responsible adult, if she is struggling herself but helps someone who lies about everything. Please tell me I'm not wrong.