Maybe ask your BO to speak to Sophies (previous) owner.
My mare is bottom of the pecking order in a herd situation but has never been hurt or injured because of it - it's just normal animal behaviour - if Sophie was a dominant mare, she'd probably have a few injuries by now fighting with the other mares in the herd.
I'd maybe see if you could offer her money for Sophie and have a proper bill of sale drawn up (do you have her passport or do you not need them where you're from) - she's probably worried for her horse and although she had offered her to you, just wants what she feels is best - hopefully the visits will start to lessen and you'll be left in peace.
Could your parents speak to her - it's all new at the moment, and probably when things settle down, hopefully she'll see that you're the best thing for Sophie.
Good luck - i hope it all works out for you xx