Failing to attend Mass because it would upset people isn’t, IMHO, a just or unavoidable reason. One needs only think of the many Christians who (in the early Church, Elizabethan England, Colonial America, modern Vietnam, etc) risked their very lives to attend Mass to see that simply striving to hide your Catholicism probably isn’t enough to abrogate your requirement to be at Mass on Sunday. Besides, you probably could have come up with some reason to be out of the house for an hour without even revealing where you where going. Remember the words of Christ Himself in Matt 10:33 “But whoever disowns me before others, I will disown before my Father in heaven.”
Or, think of it this way. There are three conditions for a sin to be mortal.
Grave Matter - missing Mass on a Sunday is grave matter. No question.
Full Consent - you were neither snowed in, nor ill, nor imprisoned, nor in a locale where there were no Masses, nor without a car, etc. Could you have gone to Mass? Did you choose not to? Did you decide that going to Mass wasn’t worth the stress? If so, that would seem to be full consent.
Full Knowledge - you knew you had to go to Mass on Sunday. You knew it was Sunday. Full Knowledge was, therefore, present.
I’m not trying to read your soul. Only you can answer the questions surrounding number 2 above. I will, however, strongly suggest you ought to see a priest before receiving Eucharist and make sure this wasn’t a mortal sin. If it was, confess it and move on. You might also speak to the priest specifically about your situation with your in-laws. Father knows best in these situations and we (myself first and foremost) can easily get into the self-justification game. Father can tell you whether you need to attend Mass on such visits or not.
God bless. I’ll say an Ave for you. :signofcross: