Contacts/messaging problems

2 days ago i noticed that one of my contacts (my sister) messaged me but it showed up as their number instead of their name, shortly after it changed back so i took little notice of it

however, since then, one of my other main conversations (my boyfriend) on iMessage has vanished from my iPhone. it is still visible on my macbook and iPad but not on my phone.

if i text my boyfriend, it sends it to my sister, no matter what i do. I've typed in his name/contact info when writing a new message and it comes up as her conversation. i typed in his number manually and it still came up as my sisters conversation.

i got him to text my number and on his iMessage it says 'delivered' but no message ever comes through. my texts don't go to him either, just to my sister.

I've deleted them as contacts and re-added them but the exact same thing happens.

please help!!! 😟

thank you