Here is a fairly elaborate example, intended for educational purposes rather than performance. It introduces some interesting Python concepts such as list comprehensions and generators, as well as a good example of recursion in which fringe cases need to be checked appropriately. Extensions, e.g. only teams with an equal number of players are valid, are easy to implement in the appropriate individual functions.
def listFairestWeakTeams(ratings):
    current_best_weak_team_rating = -1
    fairest_weak_teams = []
    for weak_team in recursiveWeakTeamGenerator(ratings):
        weak_team_rating = teamRating(weak_team, ratings)
        if weak_team_rating > current_best_weak_team_rating:
            fairest_weak_teams = []
            current_best_weak_team_rating = weak_team_rating
        if weak_team_rating == current_best_weak_team_rating:
            fairest_weak_teams.append(weak_team)
    return fairest_weak_teams
def recursiveWeakTeamGenerator(
    ratings,
    weak_team=[],
    current_applicant_index=0
):
    if not isValidWeakTeam(weak_team, ratings):
        return
    if current_applicant_index == len(ratings):
        yield weak_team
        return
    for new_team in recursiveWeakTeamGenerator(
        ratings,
        weak_team + [current_applicant_index],
        current_applicant_index + 1
    ):
        yield new_team
    for new_team in recursiveWeakTeamGenerator(
        ratings,
        weak_team,
        current_applicant_index + 1
    ):
        yield new_team
def isValidWeakTeam(weak_team, ratings):
    total_rating = sum(ratings)
    weak_team_rating = teamRating(weak_team, ratings)
    optimal_weak_team_rating = total_rating // 2
    if weak_team_rating > optimal_weak_team_rating:
        return False
    elif weak_team_rating * 2 == total_rating:
        # In case of equal strengths, player 0 is assumed
        # to be in the "weak" team
        return 0 in weak_team
    else:
        return True
def teamRating(team_members, ratings):
    return sum(memberRatings(team_members, ratings))    
def memberRatings(team_members, ratings):
    return [ratings[i] for i in team_members]
def getOpposingTeam(team, ratings):
    return [i for i in range(len(ratings)) if i not in team]
ratings = [5, 6, 2, 10, 2, 3, 4]
print("Player ratings:     ", ratings)
print("*" * 40)
for option, weak_team in enumerate(listFairestWeakTeams(ratings)):
    strong_team = getOpposingTeam(weak_team, ratings)
    print("Possible partition", option + 1)
    print("Weak team members:  ", weak_team)
    print("Weak team ratings:  ", memberRatings(weak_team, ratings))
    print("Strong team members:", strong_team)
    print("Strong team ratings:", memberRatings(strong_team, ratings))
    print("*" * 40)
Output:
Player ratings:      [5, 6, 2, 10, 2, 3, 4]
****************************************
Possible partition 1
Weak team members:   [0, 1, 2, 5]
Weak team ratings:   [5, 6, 2, 3]
Strong team members: [3, 4, 6]
Strong team ratings: [10, 2, 4]
****************************************
Possible partition 2
Weak team members:   [0, 1, 4, 5]
Weak team ratings:   [5, 6, 2, 3]
Strong team members: [2, 3, 6]
Strong team ratings: [2, 10, 4]
****************************************
Possible partition 3
Weak team members:   [0, 2, 4, 5, 6]
Weak team ratings:   [5, 2, 2, 3, 4]
Strong team members: [1, 3]
Strong team ratings: [6, 10]
****************************************