What to do?

Ollie now 21 months old has become very naughty and sneaky since his baby brother arrived.

He comes over to me when holding Joel or feeding him and he kisses his head and gently strokes him but then in a split second he changes and grabs Joel in a nasty way well I don't know if he intends for it to be nasty but he has been told off many times now and left to cry it out in his room but he still does it.

As Ollie's jealousy is increasing I'm worried Joel will be hurt, What do you do?
Has anyone else experienced this, I'd love to hear your stories and how you have overcome them or does it really just get better as people say?

I guess from Ollie's point of view he sees Joel as an intruder taking the attention mummy and daddy always gave to him and he just can't bare sharing. He's found that being naughty gets him attention but I hate shouting at him and all I seem to say is 'No Ollie', 'stop Ollie' and our punishment at the moment is putting him in his cot and leaving him there for 10-15 mins. Then explain to him what he has done.

I think now a bigger age gap would have been much easier to handle. I feel guilt towards both really as the constant telling off with Ollie and also not being able to do the same things I did with Ollie with Joel. If I put a toy in front of Joel or lay him on a play mat Ollie goes mad because he doesn't want Joel to have it and tries to snatch them away which creates another massive episode and mostly ends up with both boys crying which is not a desired sound.

Children are hard work and so stressful if another person comments that I'm now a lady of leisure I think I may punch them.