Am I wrong for not watching my nephew during weekend?

I watch my nephew ( not yet a year old)5 days a week for 10 +/- hours a day(while his parents are at work). Am I wrong for not waiting to watch him on the weekend. I have told his mom I don’t want to watch him on the weekends as well as telling her(since she was pregnant) she needed to have other back ups to watch him incase I’m sick, something comes up or it’s the weekend. I feel like she doesn’t understand or gets upset/ill with me when she ask me to watch him during a weekend day and I tell her no. (Because she said she doesn’t ask me to do it all the time) My question is…am I wrong for telling her no when she has asked me or mentions watching him on the weekend so she/they (her and spose)can have some “kid free time” (I have older kids so I fully understand needing kid free time, that is not the issue. The issue is, I have stated a few times over now I do not want to watch him during the weekend if not emergency or something like that)and this is where needing the back up sitter comes in that I have told her repeatedly before she needs to have. I love my nephew and I feel I am helping out best I can but he is a difficult baby sometimes. I have older kids and the weekend is my family’s time for resting from the week before and getting things done around the house we need to get done. Please don’t be rude but I welcome any opinions about this issue.