Read this. I won't mention some of the habits you have since they're in here; you should watch your videos and find/deal with them yourself:
http://www.reddit.com/r/SSBM/comments/2rnvkv/list_of_common_noob_habits_you_should_all_try_to/
Next time you're going to post a video here, analyse it yourself first and post the results. This is both so that you think about what you're doing and learn (thinking for yourself is really important, obviously), and so that no-one ends up pointing out things that you're already fully aware of. Also, people can go through your analysis with you which is good.
You
really need to learn techskill; it's holding you back a ridiculous amount. Go into the lab and practice. In the long term, there's no point learning how to win with your current/limited tools when you can have better tools available to you with practice (and when winning with your current tools is practically impossible anyway). I can't give you too much Fox-specific advice without seeing you play a Fox, but you'll find that once you get comfortable with your fundamental tech you'll be able to come up with solutions yourself much more readily.
Jump cancel grab is easy to learn and really important, so learn that soon; you literally just press jump and grab at the same time. Basically do it all the time, except with Sheik (because she has a useful boost grab), and sometimes against Kirby and Jigglypuff (since dash grab can grab them when they're crouching more often). You don't have to worry about that though; just jump cancel grab every time.
There aren't really any tips to learning L-cancelling, other than to remember to change your timing based on whether you hit your opponent or not, and to use the "flash red on missed L-cancel" code.
You absolutely need to dash-dance. That's how you beat aerial approaches (dash forwards, they do a short hop aerial at you, you dash away then back in and grab their landing, you kill them off the grab). Not dash-dancing is also forcing you to rely on counter to deal with a lot of approaches (it's either that or shield).
Wavedash out of shied to punish needles.
When you're on the ledge, make sure to consciously mix up your options (stand, ledgeroll, waveland, stand to buffer roll, etc.). Don't always get off the ledge immediately. Refresh your intangibility by dropping from the ledge (usually as soon as possible), fastfalling down, and then jumping back up (all while staying below the ledge so you can't be hit very easily). Sometimes drop->jump->fair if the opponent gets too close (at low percents the opponent can easily punish this from crouch cancel). Watch Mew2King.
You can't edgeguard Sheik like you're doing; you're giving up the ledge for free every time, it's just that your brother isn't going to it for some reason (tell him to do so). Basically. what you're supposed to do is grab the ledge so they can't go there (do it late so you're intangible), then react to where they teleport and waveland forward smash or whatever.
Sheik's up-B has more lading lag than you think. In the first game you countered immediately after he landed rather than taking your free punish.
Try to extend your combos more. Going for extensions when you're not quite sure whether they'll work makes you learn faster compared to always making suboptimal choices that you know will work. It's friendlies; that's your best opportunity to try this kind of thing. This is a good example of what I'm talking about; I feel like you got overexcited with your 2 hit combo and did the forward smash because you were scared of dropping it:
https://www.youtube.com/watch?v=_GIz6XK0bk8&index=2&list=PLu4VZMkd4gK9J0RdMmUx0_srEVZIV-D9c&t=56s
You should be aware of stage positioning and change your decisions accordingly. Generally, if you have an opportunity to get your opponent offstage and into an edgeguard scenario, then that's more valuable than dealing damage and resetting the situation (since the edgeguard can potentially result in a kill). Going for damage is more reliable, but edgeguards have a higher reward and in a lot of cases your chance of success can be very high for that kind of reward; see Sheik vs spacies after a back throw.
I didn't see you down tilt in those videos.