Sleeve underarm and chest too tight

Did you clip the sleeve seam allowance? That is usually what I haven't done, when that happens. Barbara in SC

From everything you have written, it sounds like you have chosen too small a pattern. It's also possible that the pattern is poorly drafted, it's rare but it does happen. Another possibility is that the style is not right for your body type. What pattern are you using?

Unless you are quite skilled at altering patterns I would not advise you to attempt an alteration to an armscye and sleeve head for a set in sleeve.

When you say, "... I took measurements of myself with the help of a friend and then proceeded to cut out a muslin ..." it's not clear how you used those measurements. Did you use your measurements to select the correct pattern size, based on the body measurement chart provided by the pattern company, or did you use those measurements to measure pattern pieces? Commercial patterns have built in ease (of anything from 1 to 5 inches) in order to fit the human body, you cannot make a garment to exact body measurements.

If the armhole is too small, the chest too tight and the back vent gapes open, that is all a pretty good indication that THE PATTERN YOU HAVE SELECTED IS TOO SMALL. Start over with a new pattern, it will be cheaper than cutting your good fabric too small.

If this were the only problem, I'd suggest considering tracing the next size armscye and sleeve or perhaps being able to get away with sewing the armscye with a larger seam allowance. However, the slit pulling open when you're relaxed makes me think the jacket really is too small for you.

Do you have a commercial jacket that fits? Can you measure and compare with the pattern you're using?

One last thing to check... are you sewing with the correct seam allowance?

Kay

Your pattern is too small. The armhole is the hardest part of a pattern to alter, because it requires altering the sleeve as well, which is not an amateur alteration. Since the chest is too tight, and it splays open in the back, it sounds like it is definitely too small. I think you must have bought a pattern based on RTW measurements, rather than what the pattern says. Disregard the sizes listed on a pattern; go by the actual measurements to get the correct size. It's always easier to take in than to let out, and you can do this in your muslin by pinching out and pinning, or slashing and spreading where needed. Then, use your muslin as the new pattern. Just make sure that you write right on the muslin the grainlines, odd pieces that may be mis-identified when taking it apart. Oh yes, and when you make a muslin, use a basting stitch to put it together, so that it's easy to rip apart when you're finished. You can then carefully press the pieces without distorting them, and use them for your pattern.

Teri