Things may get better when DD is big enough to (I don't want to use the word fight but it is Sunday evening and the best I can do ATM) fight back in a way that your DS can't get away with it and he may realise it is time to share. My DD2 went through extreme clingyness around the age of 11months to 13 months. Made it very hard at Christmas time when everyone wanted to have some of her attention. I hope it eases for you soon.

Also, before DD2 could crawl or walk we would always ask DD1 to get her a toy when she was getting one for herself and give it to her. This could be something you could try as a way of showing sharing.