Is there sexism in general in tennis sports?
Yeah. (Example at the end of my comment.)
Was sexism involved in this particular case?
Nope.
Serena broke the rules. Now she's gotta pay (quite literally, too, although her fine is less than 1% of her winnings, so...) She first got a warning for cheating (and it's pretty clear she knew her coach was instructing her because during the game, she said to the umpire he was just giving her a thumbs up, while the coach said in an interview later that she didn't see him. So which was it?) Then she broke the racket because she was losing and getting a warning, which is something anyone is going to penalised for. And then she didn't stop but continued to harass the umpire. Also, keep in mind that she didn't get penalised for just calling the umpire a thief; this was the last straw that broke the camel's back. She was arguing with him for much longer than that. When she accused him of being a liar and a thief who penalised her because he has a sexist agenda, well, that's when the umpire got tired of her bickering. And do keep in mind that Ramos, the umpire who penalised Serena, issued code violations to other players for much less than this, including men (e.g. Andy Murray and Nick Kyrgios who ironically also accused Ramos of double standards.) No sexism here. Ramos is simply stricter than other umpires.
Don't want to get penalised? Don't. Break. The. Rules. Didn't think I had to spell that one out, but here we are.
Also, with regards to the woman who took off her top, I think you're talking about the female player who took off her top just over a week ago because she was wearing it inside out, so she wanted to fix that? Is that the player you're talking about? If so, the penalty was completely unjustified, and yes, that would be an example of sexism (which is why the rule was changed immediately after this.) I think it's more accurate to say, though, that she adjusted her shirt, not that she took it off. The latter doesn't imply that she put it on again literally after a few seconds, which she did. She only took it off because she was wearing it inside out, so she fixed it and put it back on.