Does not sound good. The best thing is check the rear jacing point on the same side, if it's sound, jack from that point. You will find a heap of drain holes along the bottom of the sill. Clean them out first with a bit of wire and start hosing out the sill. See what nasties come out. Once it's clean use a thin pointed metal scribe and gently start poking around the front jack point to establish the integrity of the steel and how far the rust has gone. You may have caught it in time. If you start digging madly and it's serious, you could create unnecessary work for yourself trying to hide it before you trade it or off load it. Go slowly, it's probably not too bad, a weak spot for rust in these areas. Check both sills and get an idea of their state first , then make a decision or have a panel beater quote for repairs. Sorry.