"My little sister loves playing soccer with her friends the most."
Well, if you were to take out をする, then you'd also need to take out the nominaliser の too. But I think the sentence you'd end up with would have quite a different meaning...
- 友だち = friends
- サッカー = soccer (US) / football (non-US)
- サッカーをするの = playing soccer
友だちとサッカー = "friends and soccer"
友だちとサッカーをするの = "playing soccer with friends"
Here's what the full sentence is like without をするの...
"My little sister likes her friends and soccer the most."
I don't know whether it even makes sense in Japanese to say いちばん好きです about two things instead of one. It might be okay though.(?)
I think another issue is that this doesn't say what she likes about soccer. From this sentence we can't tell if she has ever even played soccer before (and she might not want to ever play it either). What she could mean is simply that she really likes watching professional soccer matches on TV (and can thus say she like "soccer" even if she's never played it herself). ^^