Even though you think you and your sister are both nobs.......
which I highly doubt. Getting her to come out and give you a hand is a good way to go.
If you determine that he's not uncomfortable from the mishap by lunging him lightly and carefully watching how he's moving, then what Sonny has said is the next step. Start at square one the way you would with a horse that has never had a saddle on them before.
All the while have your sister holding a lead rope with her standing at his side, not in front of him. If he starts to take off you can stop him with the reins and she can try to spin him back around and stop him. With double pressure you should get him stopped. If this happens go back to step one.
Once your on and feeling pretty good have you sister walk along with you still holding the lead rope.
NEVER. EVER tie a horse to mount them. There is not one good reason to do this, EVER.