Feel the need to give up babysitting niece?

I have been babysitting my 4 mth old niece .....but I have a 12yr old, 5yr old, and 2 1/2 yr old..... My kids love having her here to play with and love on.... but my delimma is I feel that I cannot do all the stuff with my kids( like playing ball, going to the park, having our tea parties and picnis outside) not to mention how hard it is getting 2 to dance 3 days aweek and helping my son with his show calfs , swimming lessons..etc... I love having a baby especially my niece .... but she does not like to be put down to sleep or even kick and play( her mom carries her in a sling all day ( she holds her thru her daily naps, puts her in her sling to do chores) I know that that is her mothers choice and , she is a great mom.... but i cannot hold her all the time.... she will cry the minute you try to put her down...My question.....Her parents both work and I plan on telling them this weekend that I will not be able to keep her starting the first of august.... is this plenty of notice ?