In the neutral game, Sheik is generally going to camp you while pressuring you to try to push you into a corner where you can't move around much. She will then use her tilts or select aerials to stop your aerial approaches from the corner, and either combo you, get you into a ledgeguard situatation, or get you locked into tech chases. You should mix up your DI, mix up your techs, and generally recover medium-low while trying to wall-tech if they d-smash. When you do get an opportunity on Sheik, make it matter. Combo her hard. For edgeguarding, make her land on stage and stomp/knee. Do not let needles lock you down.
Luigi interestingly often plays a lot like Falcon, abusing massive movement to try to get grabs, tech chases or select hits. When he finally hits you, he's going to combo you upwards and then try to get you into an edgeguarding situation, so DI away (but not to off stage.) Luigi has a lot of trouble with nairs, knees, uairs, and stomps, so try to chase Luigi while he's in hitstun for your combos into knee. Do not let Luigi's tornado (down-b) throw you off in neutral, and know that you can/should punish it, and do not get naired out of combos.
