21 month daughter still plays with my breasts... How do I stop it?

I breast fed my daughter for 14 months and now she is 21 months and she feels the need to stick her hand down my shirt. It never fails she is always going for them to cuddle next to and play with them. How can I stop this before she gets too old??? Also at night if I don't let her have them she crys.... Not to mention I'm 9 weeks pregnant and plan on breast feeding my next child. I don't want her to get mad because her little brother or sister is on them. Any advice???