Someone help me.?

This woman keeps harassing my sister for many years (over my sisters baby dad. My sister left her baby dad years ago when she found out he has been cheating on her with this woman who’s been harassing her. The woman & my sisters baby dad are engaged & she harass my sister because she’s obsessed with my niece .. literally ) my sister got this text today & we are trying to locate the number & see who owns it.

the woman has stalked my sister before to know where she lives & now my sister moved again a few weeks ago & now I’m 90% sure she’s starting to harass her again.

Can someone help me try to see what I can do with this number.?? How to locate it or something .. (the woman has 3 daughters with him now all under 3/4 & she won’t stop harassing my sister & she won’t stop being obsessed with my niece)