The suffix -(t)er shows up in different places in the IE lgs. Comparative versus nomen agentis suffix. Is the -ter in father, mother, brother different? or a different use. What about in dexter and sinister. About in-: it can be used in the sense of 'in(side of) / within' something, 'not' something (cf. our un-, and an intensifier as inflammable (cf. re- elsewhere in this thread).