What is the fist step and proccess for making a family member get medical help and applying for guardianship?

My 53 yr old sister is intellectually disabled & has the mental capacity of a 12-13 yr old. There is also basic common sense & logic that she doesn’t understand. She has refused for yrs to go to the dr for reg. routine checkups. As she gets older her mental disability gradually worsens. In the past 6 wks she met a guy, kept it secret from family & began moving her things to his home, says they’re getting married & plans on leaving state with him in a few wks. She is also lying to us about where she is when she’ll be home & deleting her texts from him off of her phone (even though no one has attempted to read them). We have repeatedly voiced our concerns to her & her boyfriend to no avail. What’s worst is I have another disable sister and they live together alone. They both have they’re strengths & weaknesses that fit together so they were able to take care of each other until now. My sister will lie to my other sister saying she’s staying home wait til she’s asleep & leave.