One week ago exactly I introduced our lonely Tamworth sow to two new 9 week old OSB weaners, having first made up a small temporary ark knowing they wouldn't be allowed into the main one to start with. For the first 2 days there was constant chasing by Mango and attempted biting, but both weaners got out of the way too fast. For their part, they tried and tried to make friends right from the beginning, and initially chose to sleep outside behind the ark, rather than in the spare one.
Gradually hostilities lessened and have now vanished completely. On the third night the OSBs slept in one corner of the ark and Mango at the opposite far end, yesterday and tonight all three are cuddled up together as if they have known each other for ever. The only thing I am still doing is separating them for feeds, as Mango wolfs hers down in 1/3 of the time the two little ones take.
I'm sure it is only a matter of time and yours will become the greatest friends too. After 4 or 5 days the bigger pig will probably have completely forgotten what life was like when they had a different companion, or were on their own, and from then on a new status quo evolves where the new group happily co-exist. Good luck, and don't worry - Tamsaddle