I couldn't think how to title this post, but you should get some idea!

Esme finally came into season, almost a year since the last one, when she had her first litter (she is usually around 8 months). Now the problem we are having is that Mo is constantly humping her, and if she stops Esme is pestering her, flagging and shoving her bum in her face. Both dogs are exhausting themselves. Previous seasons Esme has pestered Hamish (neutered male) but not to this extent.

Should I be stopping it, and keeping them apart or just ignore it and let them get on with it? Any advice gratefully received. Certainly not had this problem with any of the others.

I'm so glad I kept a bitch from the litter and not a dog!!!

If they're exhausting each other I would separate them so they can get some rest for a bit :-)

In my limited experience in this matter, I think this is excitement/learned behaviour.  They obviously did it to 'practice' and found it to be a great game so now do it all the time.  I would 'un-learn' the fun.  Don't give anyone a hard time about it, just re-direct the game.  Get a toy and join in! But if anyone mounts anyone else, just pull them off.  Also, if you don't want to up the excitement, stop the play altogether.  Just move the dog ontop off and take it out the room for a time out to calm down.  If when you take her back in, she does it again, just turn around and take her out again.  They will soon learn that this behaviour gets them taken out the fold and will stop.  You need to be consistant(as with all things doggie).  You don't want to make a big deal or give anyone a row or anything as you may still want one or both of the girls to mate one day!lol.

It also appears that Mum has bought Daughter in and they are both in season now (although the youngster isn't swollen at all and it is her first season). All quiet at the moment....